diff --git a/packages/studio/src/App.tsx b/packages/studio/src/App.tsx
index ffafec908..520b4e855 100644
--- a/packages/studio/src/App.tsx
+++ b/packages/studio/src/App.tsx
@@ -399,7 +399,7 @@ export function StudioApp() {
   } = useInspectorState(
     panelLayout.rightPanelTab,
     panelLayout.rightInspectorPanes,
-    panelLayout.rightCollapsed,
+    panelLayout.effectiveRightCollapsed,
     isPlaying,
     domEditSession.domEditSelection,
     gestureState === "recording",
@@ -512,7 +512,7 @@ export function StudioApp() {
                       />
                     }
                     right={
-                      panelLayout.rightCollapsed ? null : (
+                      panelLayout.effectiveRightCollapsed ? null : (
                         <StudioRightPanel
                           designPanelActive={designPanelActive}
                           activeBlockParams={activeBlockParams}
diff --git a/packages/studio/src/components/StudioHeader.test.ts b/packages/studio/src/components/StudioHeader.test.ts
new file mode 100644
index 000000000..4d897faa7
--- /dev/null
+++ b/packages/studio/src/components/StudioHeader.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { shouldOpenInspector } from "./StudioHeader";
+
+describe("shouldOpenInspector", () => {
+  it("opens when the panel is hidden", () => {
+    expect(shouldOpenInspector(true, false)).toBe(true);
+  });
+
+  it("opens when a non-inspector tab is showing", () => {
+    expect(shouldOpenInspector(false, false)).toBe(true);
+  });
+
+  it("closes when the inspector is genuinely on screen", () => {
+    expect(shouldOpenInspector(false, true)).toBe(false);
+  });
+
+  it("opens when the window railed the panel away", () => {
+    // The regression this guards: the button used to branch on the raw
+    // rightCollapsed intent, which is still `false` while the window has the
+    // panel railed. That took the close branch, wrote rightCollapsed=true, and
+    // since that value is synced into the shareable Studio URL, a click that
+    // did nothing visible rewrote the link.
+    const userIntentIsOpen = false;
+    const windowRailedItAway = true;
+    expect(shouldOpenInspector(windowRailedItAway, true)).toBe(true);
+    expect(shouldOpenInspector(userIntentIsOpen, true)).toBe(false);
+  });
+});
diff --git a/packages/studio/src/components/StudioHeader.tsx b/packages/studio/src/components/StudioHeader.tsx
index 6154a4d33..0e9855f82 100644
--- a/packages/studio/src/components/StudioHeader.tsx
+++ b/packages/studio/src/components/StudioHeader.tsx
@@ -196,6 +196,21 @@ export function ViewModeToggle() {
   );
 }
 
+/**
+ * Does the header's Inspector button open the panel, or close it?
+ *
+ * Takes the EFFECTIVE collapse state, so a panel the window has railed away
+ * counts as closed even though the user's stored intent still says open. The
+ * argument name is the guard: passing raw intent here is the bug this exists
+ * to keep out.
+ */
+export function shouldOpenInspector(
+  effectiveRightCollapsed: boolean,
+  inspectorPanelActive: boolean,
+): boolean {
+  return effectiveRightCollapsed || !inspectorPanelActive;
+}
+
 // fallow-ignore-next-line complexity
 export function StudioHeader({
   captureFrameHref,
@@ -208,7 +223,11 @@ export function StudioHeader({
   onExport,
 }: StudioHeaderProps) {
   const { projectId, editHistory, handleUndo, handleRedo, renderQueue } = useStudioShellContext();
-  const { rightCollapsed, setRightCollapsed, setRightPanelTab } = usePanelLayoutContext();
+  // effectiveRightCollapsed, not the raw intent: in the auto-railed state the
+  // intent is still "open" while the panel is hidden, so branching on intent
+  // made this button write rightCollapsed=true — and that value is synced into
+  // the shareable Studio URL, so a dead click would rewrite a link.
+  const { effectiveRightCollapsed, setRightCollapsed, setRightPanelTab } = usePanelLayoutContext();
   const isRendering = renderQueue.isRendering;
 
   return (
@@ -328,7 +347,7 @@ export function StudioHeader({
           <button
             type="button"
             onClick={() => {
-              if (rightCollapsed || !inspectorPanelActive) {
+              if (shouldOpenInspector(effectiveRightCollapsed, inspectorPanelActive)) {
                 trackStudioEvent("panel_toggle", { panel: "inspector", collapsed: false });
                 setRightPanelTab("design");
                 setRightCollapsed(false);
diff --git a/packages/studio/src/components/StudioLeftSidebar.tsx b/packages/studio/src/components/StudioLeftSidebar.tsx
index 4ca96ee69..0cb90a5fc 100644
--- a/packages/studio/src/components/StudioLeftSidebar.tsx
+++ b/packages/studio/src/components/StudioLeftSidebar.tsx
@@ -36,7 +36,7 @@ export function StudioLeftSidebar({
   onAddCompositionToTimeline,
 }: StudioLeftSidebarProps) {
   const {
-    leftCollapsed,
+    effectiveLeftCollapsed,
     leftWidth,
     adjustPanelWidth,
     toggleLeftSidebar,
@@ -71,7 +71,7 @@ export function StudioLeftSidebar({
     [renderQueue, waitForPendingDomEditSaves],
   );
 
-  if (leftCollapsed) {
+  if (effectiveLeftCollapsed) {
     return (
       <div className="mr-0.5 flex w-10 flex-shrink-0 flex-col items-center rounded-lg border border-neutral-800/50 bg-neutral-950 pt-1">
         <button
diff --git a/packages/studio/src/components/nle/NLEContext.tsx b/packages/studio/src/components/nle/NLEContext.tsx
index 8874acb80..5f5ae4293 100644
--- a/packages/studio/src/components/nle/NLEContext.tsx
+++ b/packages/studio/src/components/nle/NLEContext.tsx
@@ -11,7 +11,7 @@ import { useTimelinePlayer, usePlayerStore } from "../../player";
 import type { TimelineElement } from "../../player";
 import type { CompositionLevel } from "./CompositionBreadcrumb";
 import { useCompositionStack } from "./useCompositionStack";
-import { MIN_TIMELINE_H, MIN_PREVIEW_H } from "./TimelineResizeDivider";
+import { MIN_TIMELINE_H, fitTimelineHeight } from "../../utils/fitPanels";
 import { setCompositionSourceMap } from "../editor/domEditingDom";
 import { ensureMotionPathPluginLoaded } from "../../utils/gsapSoftReload";
 import { readStudioUiPreferences, writeStudioUiPreferences } from "../../utils/studioUiPreferences";
@@ -273,13 +273,22 @@ export function NLEProvider({
   }, []);
   const containerRef = useRef<HTMLDivElement>(null);
   // A height persisted on a tall window can exceed this window's container and
-  // collapse the flex-1 preview to 0px — clamp once the container is measurable
-  // (the drag/keyboard paths already clamp; the restore path must too).
+  // collapse the flex-1 preview to 0px. Observing the container rather than
+  // clamping once at mount is what makes a window RESIZED after load behave the
+  // same as one loaded at that size: dragging 760 -> 520 tall used to leave the
+  // timeline at its stored 429px and the preview at 47px.
   useEffect(() => {
-    const containerH = containerRef.current?.getBoundingClientRect().height;
-    if (!containerH) return;
-    const max = containerH - MIN_PREVIEW_H;
-    setTimelineH((prev) => (prev > max ? Math.max(MIN_TIMELINE_H, max) : prev));
+    const element = containerRef.current;
+    if (!element || typeof ResizeObserver === "undefined") return;
+    const reconcile = () => {
+      const containerH = element.getBoundingClientRect().height;
+      if (!containerH) return;
+      setTimelineH((prev) => fitTimelineHeight(containerH, prev));
+    };
+    reconcile();
+    const observer = new ResizeObserver(reconcile);
+    observer.observe(element);
+    return () => observer.disconnect();
   }, []);
 
   const hasLoadedOnceRef = useRef(false);
diff --git a/packages/studio/src/components/nle/TimelineResizeDivider.tsx b/packages/studio/src/components/nle/TimelineResizeDivider.tsx
index 30a462c71..c1c48d027 100644
--- a/packages/studio/src/components/nle/TimelineResizeDivider.tsx
+++ b/packages/studio/src/components/nle/TimelineResizeDivider.tsx
@@ -1,7 +1,5 @@
 import { useCallback, useRef } from "react";
-
-export const MIN_TIMELINE_H = 100;
-export const MIN_PREVIEW_H = 120;
+import { MIN_PREVIEW_H, MIN_TIMELINE_H, fitTimelineHeight } from "../../utils/fitPanels";
 
 /**
  * Horizontal drag/keyboard-resizable divider between the preview and the
@@ -41,12 +39,7 @@ export function TimelineResizeDivider({
       if (!isDragging.current || !containerRef.current) return;
       const rect = containerRef.current.getBoundingClientRect();
       const mouseY = e.clientY - rect.top;
-      const containerH = rect.height;
-      const newTimelineH = Math.max(
-        MIN_TIMELINE_H,
-        Math.min(containerH - MIN_PREVIEW_H, containerH - mouseY),
-      );
-      setTimelineH(newTimelineH);
+      setTimelineH(fitTimelineHeight(rect.height, rect.height - mouseY));
     },
     [disabled, containerRef, setTimelineH],
   );
@@ -61,10 +54,10 @@ export function TimelineResizeDivider({
       if (disabled) return;
       if (e.key !== "ArrowUp" && e.key !== "ArrowDown") return;
       e.preventDefault();
-      const containerH = containerRef.current?.getBoundingClientRect().height ?? Infinity;
+      const containerH = containerRef.current?.getBoundingClientRect().height ?? 0;
       const delta = e.key === "ArrowUp" ? 16 : -16;
       setTimelineH((prev) => {
-        const next = Math.max(MIN_TIMELINE_H, Math.min(containerH - MIN_PREVIEW_H, prev + delta));
+        const next = fitTimelineHeight(containerH, prev + delta);
         persistTimelineH(next);
         return next;
       });
diff --git a/packages/studio/src/contexts/PanelLayoutContext.tsx b/packages/studio/src/contexts/PanelLayoutContext.tsx
index b696da0a8..6666fccbc 100644
--- a/packages/studio/src/contexts/PanelLayoutContext.tsx
+++ b/packages/studio/src/contexts/PanelLayoutContext.tsx
@@ -17,9 +17,10 @@ export function PanelLayoutProvider({
     rightWidth,
     adjustPanelWidth,
     leftCollapsed,
-    setLeftCollapsed,
     rightCollapsed,
     setRightCollapsed,
+    effectiveLeftCollapsed,
+    effectiveRightCollapsed,
     rightPanelTab,
     setRightPanelTab,
     rightInspectorPanes,
@@ -41,9 +42,10 @@ export function PanelLayoutProvider({
       rightWidth,
       adjustPanelWidth,
       leftCollapsed,
-      setLeftCollapsed,
       rightCollapsed,
       setRightCollapsed,
+      effectiveLeftCollapsed,
+      effectiveRightCollapsed,
       rightPanelTab,
       setRightPanelTab,
       rightInspectorPanes,
@@ -59,9 +61,10 @@ export function PanelLayoutProvider({
       rightWidth,
       adjustPanelWidth,
       leftCollapsed,
-      setLeftCollapsed,
       rightCollapsed,
       setRightCollapsed,
+      effectiveLeftCollapsed,
+      effectiveRightCollapsed,
       rightPanelTab,
       setRightPanelTab,
       rightInspectorPanes,
diff --git a/packages/studio/src/hooks/usePanelLayout.test.ts b/packages/studio/src/hooks/usePanelLayout.test.ts
index 0a3d18a7c..014a96c74 100644
--- a/packages/studio/src/hooks/usePanelLayout.test.ts
+++ b/packages/studio/src/hooks/usePanelLayout.test.ts
@@ -60,6 +60,11 @@ function renderPanelLayout() {
   return renderPanelLayoutWith(usePanelLayout);
 }
 
+function resizeWindowTo(width: number) {
+  Object.defineProperty(window, "innerWidth", { configurable: true, value: width });
+  window.dispatchEvent(new Event("resize"));
+}
+
 describe("usePanelLayout — right inspector panes", () => {
   it("opens Design with the intended viewport-scaled panel widths", () => {
     const harness = renderPanelLayout();
@@ -160,6 +165,118 @@ describe("usePanelLayout — right inspector panes", () => {
     harness.unmount();
   });
 
+  it("caps a panel relative to the window instead of at a flat 600px", () => {
+    resizeWindowTo(700);
+    const harness = renderPanelLayout();
+    // The old flat cap let the inspector claim 600 of a 700px window.
+    expect(harness.getState().rightWidth).toBeLessThanOrEqual(280);
+    harness.unmount();
+  });
+
+  it("rails both panels once the window cannot fit them", () => {
+    resizeWindowTo(560);
+    const harness = renderPanelLayout();
+    expect(harness.getState()).toMatchObject({
+      effectiveLeftCollapsed: true,
+      effectiveRightCollapsed: true,
+      leftCollapsed: false,
+      rightCollapsed: false,
+    });
+    harness.unmount();
+  });
+
+  it("auto-collapse never writes the user's persisted or URL-synced intent", () => {
+    const harness = renderPanelLayout();
+    act(() => resizeWindowTo(560));
+
+    expect(harness.getState().effectiveLeftCollapsed).toBe(true);
+    // localStorage carries leftCollapsed; the shareable URL carries rightCollapsed.
+    // A ten-second window drag must rewrite neither.
+    expect(readStudioUiPreferences().leftCollapsed).toBeUndefined();
+    expect(harness.getState().leftCollapsed).toBe(false);
+    expect(harness.getState().rightCollapsed).toBe(false);
+    harness.unmount();
+  });
+
+  it("returns the user's own width when the window grows back", () => {
+    const harness = renderPanelLayout();
+    const wide = harness.getState().leftWidth;
+
+    act(() => resizeWindowTo(560));
+    expect(harness.getState().leftWidth).toBeLessThan(wide);
+
+    act(() => resizeWindowTo(1496));
+    expect(harness.getState().leftWidth).toBe(wide);
+    harness.unmount();
+  });
+
+  it("keeps an explicitly collapsed sidebar collapsed after a narrow trip", () => {
+    const harness = renderPanelLayout();
+    act(() => harness.getState().toggleLeftSidebar());
+    expect(readStudioUiPreferences().leftCollapsed).toBe(true);
+
+    act(() => resizeWindowTo(560));
+    act(() => resizeWindowTo(1496));
+
+    expect(harness.getState().effectiveLeftCollapsed).toBe(true);
+    harness.unmount();
+  });
+
+  it("lets the user reopen a panel the window auto-collapsed", () => {
+    const harness = renderPanelLayout();
+    act(() => resizeWindowTo(560));
+    expect(harness.getState().effectiveRightCollapsed).toBe(true);
+
+    // Without this the header Inspector button would be dead below 700px.
+    act(() => harness.getState().setRightCollapsed(false));
+    expect(harness.getState().effectiveRightCollapsed).toBe(false);
+    harness.unmount();
+  });
+
+  it("opens the sidebar when the rail's own button is clicked", () => {
+    const harness = renderPanelLayout();
+    act(() => resizeWindowTo(560));
+    expect(harness.getState().effectiveLeftCollapsed).toBe(true);
+
+    // Regression: the toggle used to flip stored INTENT, which was already
+    // false here, so the click persisted leftCollapsed=true and the rail stayed
+    // railed — a dead button that silently saved a collapse nobody asked for.
+    act(() => harness.getState().toggleLeftSidebar());
+
+    expect(harness.getState().effectiveLeftCollapsed).toBe(false);
+    expect(harness.getState().leftCollapsed).toBe(false);
+    expect(readStudioUiPreferences().leftCollapsed).toBe(false);
+    // And it gets a real width: rendering an expanded sidebar at the 42px rail
+    // width would squash its own content. Only a real-UI click caught this.
+    expect(harness.getState().leftWidth).toBeGreaterThanOrEqual(200);
+    harness.unmount();
+  });
+
+  it("closes the sidebar again on the next click", () => {
+    const harness = renderPanelLayout();
+    act(() => resizeWindowTo(560));
+    act(() => harness.getState().toggleLeftSidebar());
+    act(() => harness.getState().toggleLeftSidebar());
+
+    expect(harness.getState().effectiveLeftCollapsed).toBe(true);
+    expect(readStudioUiPreferences().leftCollapsed).toBe(true);
+    harness.unmount();
+  });
+
+  it("forgets that reopen once the window is wide again", () => {
+    const harness = renderPanelLayout();
+    act(() => resizeWindowTo(560));
+    act(() => harness.getState().setRightCollapsed(false));
+    expect(harness.getState().effectiveRightCollapsed).toBe(false);
+
+    // Widening past the threshold clears the override, so a later narrow trip
+    // rails again rather than staying open forever off one old click.
+    act(() => resizeWindowTo(1496));
+    act(() => resizeWindowTo(560));
+    expect(harness.getState().effectiveRightCollapsed).toBe(true);
+    harness.unmount();
+  });
+
   it("setRightPanelTab is flat-aware: exclusivity holds for callers other than a direct in-panel tab click", async () => {
     vi.resetModules();
     vi.doMock("../components/editor/manualEditingAvailability", async () => {
diff --git a/packages/studio/src/hooks/usePanelLayout.ts b/packages/studio/src/hooks/usePanelLayout.ts
index cc358b6e6..44d5992de 100644
--- a/packages/studio/src/hooks/usePanelLayout.ts
+++ b/packages/studio/src/hooks/usePanelLayout.ts
@@ -1,4 +1,4 @@
-import { useState, useCallback, useRef } from "react";
+import { useState, useCallback, useRef, useEffect } from "react";
 import type {
   RightInspectorPane,
   RightInspectorPanes,
@@ -7,6 +7,14 @@ import type {
 import { readStudioUiPreferences, writeStudioUiPreferences } from "../utils/studioUiPreferences";
 import { trackStudioEvent } from "../utils/studioTelemetry";
 import { STUDIO_FLAT_INSPECTOR_ENABLED } from "../components/editor/manualEditingAvailability";
+import {
+  defaultPanelWidths,
+  fitPanelWidths,
+  railsEngaged,
+  type PanelWidths,
+} from "../utils/fitPanels";
+
+const NO_OVERRIDE = { left: false, right: false } as const;
 
 export interface InitialPanelLayoutState {
   rightCollapsed?: boolean | null;
@@ -20,30 +28,27 @@ function getInitialRightInspectorPanes(tab?: RightPanelTab | null): RightInspect
   return { layers: false, design: true };
 }
 
-function getInitialPanelWidths(): { left: number; right: number } {
-  const viewportWidth = typeof window === "undefined" ? 1496 : window.innerWidth;
+function readViewportWidth(): number {
+  return typeof window === "undefined" ? 1496 : window.innerWidth;
+}
+
+/**
+ * What the user WANTS each panel to be, before the window gets a say. Stored
+ * preferences win over the width-derived defaults; neither is clamped here —
+ * `fitPanelWidths` owns every clamp so there is one place that decides.
+ */
+function getPreferredPanelWidths(): PanelWidths {
   const preferences = readStudioUiPreferences();
-  const leftDefault = Math.max(240, Math.min(384, Math.round(viewportWidth * 0.257)));
-  const rightDefault = Math.max(320, Math.min(424, Math.round(viewportWidth * 0.284)));
+  const defaults = defaultPanelWidths(readViewportWidth());
   return {
-    left: Math.max(
-      160,
-      Math.min(Math.floor(viewportWidth * 0.5), preferences.leftWidth ?? leftDefault),
-    ),
-    right: Math.max(160, Math.min(600, preferences.rightWidth ?? rightDefault)),
+    left: preferences.leftWidth ?? defaults.left,
+    right: preferences.rightWidth ?? defaults.right,
   };
 }
 
-function clampPanelWidth(side: PanelSide, width: number): number {
-  const max = side === "left" ? Math.floor(window.innerWidth * 0.5) : 600;
-  return Math.max(160, Math.min(max, width));
-}
-
 export function usePanelLayout(initialState?: InitialPanelLayoutState) {
-  const [initialPanelWidths] = useState(getInitialPanelWidths);
-  const [leftWidth, setLeftWidth] = useState(initialPanelWidths.left);
-  const [rightWidth, setRightWidth] = useState(initialPanelWidths.right);
-  const panelWidthsRef = useRef(initialPanelWidths);
+  const [preferredWidths, setPreferredWidths] = useState(getPreferredPanelWidths);
+  const [viewportWidth, setViewportWidth] = useState(readViewportWidth);
   const [leftCollapsed, setLeftCollapsed] = useState(
     () => readStudioUiPreferences().leftCollapsed ?? false,
   );
@@ -54,41 +59,109 @@ export function usePanelLayout(initialState?: InitialPanelLayoutState) {
   const [rightInspectorPanes, setRightInspectorPanes] = useState<RightInspectorPanes>(() =>
     getInitialRightInspectorPanes(initialState?.rightPanelTab),
   );
+  // Set when the user explicitly reopens a panel the window had auto-collapsed,
+  // so the rail cannot immediately swallow it again. Cleared once the window is
+  // wide enough that auto-collapse is no longer in play.
+  const [autoCollapseOverride, setAutoCollapseOverride] = useState<{
+    left: boolean;
+    right: boolean;
+  }>(NO_OVERRIDE);
+
+  // Reconciliation is a live window resize away, not a mount-time snapshot: a
+  // Studio loaded at 1440 and dragged to a half-screen used to keep its pixel
+  // widths and squeeze the preview to nothing.
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    const handleResize = () => {
+      const width = window.innerWidth;
+      setViewportWidth(width);
+      // Cleared here rather than in an effect watching derived state: the rail
+      // flags depend only on width, and width only changes in this handler.
+      if (!railsEngaged(width)) {
+        setAutoCollapseOverride((prev) => (prev.left || prev.right ? NO_OVERRIDE : prev));
+      }
+    };
+    window.addEventListener("resize", handleResize);
+    return () => window.removeEventListener("resize", handleResize);
+  }, []);
+
+  const fitted = fitPanelWidths(viewportWidth, preferredWidths, autoCollapseOverride);
+  const leftCollapsedByWidth = fitted.autoCollapseLeft;
+  const rightCollapsedByWidth = fitted.autoCollapseRight;
+
+  // Rendered widths, which the drag handles measure from so the seam does not
+  // jump when a panel is currently narrower than its stored preference.
+  const fittedRef = useRef(fitted);
+  fittedRef.current = fitted;
+
   const panelDragRef = useRef<{
     side: PanelSide;
     startX: number;
     startW: number;
   } | null>(null);
 
-  const updatePanelWidth = useCallback((side: PanelSide, width: number) => {
-    const next = clampPanelWidth(side, width);
-    panelWidthsRef.current[side] = next;
-    if (side === "left") setLeftWidth(next);
-    else setRightWidth(next);
+  // Preferred widths are also held in a ref so a burst of pointer moves or
+  // keyboard nudges inside one React batch accumulates, instead of every call
+  // in the batch reading the same pre-render value.
+  const preferredRef = useRef(preferredWidths);
+
+  const setPreferred = useCallback((side: PanelSide, width: number) => {
+    const next = Math.max(0, Math.round(width));
+    preferredRef.current = { ...preferredRef.current, [side]: next };
+    setPreferredWidths(preferredRef.current);
     return next;
   }, []);
 
+  /** Transient: moves the panel without touching the stored preference. */
+  const updatePanelWidth = useCallback(
+    (side: PanelSide, width: number) => {
+      setPreferred(side, width);
+    },
+    [setPreferred],
+  );
+
+  /**
+   * Durable: only an explicit drag or keyboard nudge writes a preference. A
+   * width the window forced on us is never persisted, so the user's real
+   * preference survives a temporary squeeze and returns when the window grows.
+   */
   const commitPanelWidth = useCallback(
     (side: PanelSide, width: number) => {
-      const next = updatePanelWidth(side, Math.round(width));
-      writeStudioUiPreferences(side === "left" ? { leftWidth: next } : { rightWidth: next });
+      // Persist what the window will actually allow, not a raw pointer delta.
+      const candidate = { ...preferredRef.current, [side]: Math.max(0, Math.round(width)) };
+      const settled = fitPanelWidths(readViewportWidth(), candidate)[side];
+      setPreferred(side, settled);
+      writeStudioUiPreferences(side === "left" ? { leftWidth: settled } : { rightWidth: settled });
     },
-    [updatePanelWidth],
+    [setPreferred],
   );
 
   const adjustPanelWidth = useCallback(
     (side: PanelSide, delta: number) => {
-      commitPanelWidth(side, panelWidthsRef.current[side] + delta);
+      commitPanelWidth(side, preferredRef.current[side] + delta);
     },
     [commitPanelWidth],
   );
 
+  // The toggle acts on what the user can SEE, not on stored intent. Toggling
+  // stored intent instead made the rail's "Show sidebar" button dead in the
+  // auto-collapsed state: intent was already false, so the click flipped it to
+  // true (persisting a collapse the user never asked for) while the rail stayed
+  // railed and nothing visibly happened.
+  const effectiveLeftCollapsedRef = useRef(false);
+  effectiveLeftCollapsedRef.current = leftCollapsed || leftCollapsedByWidth;
+
   const toggleLeftSidebar = useCallback(() => {
-    setLeftCollapsed((collapsed) => {
-      writeStudioUiPreferences({ leftCollapsed: !collapsed });
-      trackStudioEvent("panel_toggle", { panel: "left_sidebar", collapsed: !collapsed });
-      return !collapsed;
-    });
+    const next = !effectiveLeftCollapsedRef.current;
+    setLeftCollapsed(next);
+    writeStudioUiPreferences({ leftCollapsed: next });
+    trackStudioEvent("panel_toggle", { panel: "left_sidebar", collapsed: next });
+    if (!next) setAutoCollapseOverride((prev) => ({ ...prev, left: true }));
+  }, []);
+
+  const setRightCollapsedWithOverride = useCallback((collapsed: boolean) => {
+    setRightCollapsed(collapsed);
+    if (!collapsed) setAutoCollapseOverride((prev) => ({ ...prev, right: true }));
   }, []);
 
   const handlePanelResizeStart = useCallback((side: PanelSide, e: React.PointerEvent) => {
@@ -97,7 +170,7 @@ export function usePanelLayout(initialState?: InitialPanelLayoutState) {
     panelDragRef.current = {
       side,
       startX: e.clientX,
-      startW: panelWidthsRef.current[side],
+      startW: fittedRef.current[side],
     };
   }, []);
 
@@ -113,7 +186,7 @@ export function usePanelLayout(initialState?: InitialPanelLayoutState) {
 
   const handlePanelResizeEnd = useCallback(() => {
     const side = panelDragRef.current?.side;
-    if (side) commitPanelWidth(side, panelWidthsRef.current[side]);
+    if (side) commitPanelWidth(side, preferredRef.current[side]);
     panelDragRef.current = null;
   }, [commitPanelWidth]);
 
@@ -158,13 +231,22 @@ export function usePanelLayout(initialState?: InitialPanelLayoutState) {
   }, []);
 
   return {
-    leftWidth,
-    rightWidth,
+    leftWidth: fitted.left,
+    rightWidth: fitted.right,
     adjustPanelWidth,
+    /**
+     * User intent. Persisted to localStorage; never written by auto-collapse.
+     * Deliberately read-only outside this hook: `toggleLeftSidebar` is the only
+     * writer, so it cannot be flipped without also clearing the rail override
+     * (which would open the sidebar and then immediately rail it again).
+     */
     leftCollapsed,
-    setLeftCollapsed,
+    /** User intent. Synced into the shareable URL; never written by auto-collapse. */
     rightCollapsed,
-    setRightCollapsed,
+    setRightCollapsed: setRightCollapsedWithOverride,
+    /** What the shell actually renders: intent OR the window forcing a rail. */
+    effectiveLeftCollapsed: leftCollapsed || leftCollapsedByWidth,
+    effectiveRightCollapsed: rightCollapsed || rightCollapsedByWidth,
     rightPanelTab,
     setRightPanelTab: trackedSetRightPanelTab,
     rightInspectorPanes,
diff --git a/packages/studio/src/utils/fitPanels.test.ts b/packages/studio/src/utils/fitPanels.test.ts
new file mode 100644
index 000000000..39a065616
--- /dev/null
+++ b/packages/studio/src/utils/fitPanels.test.ts
@@ -0,0 +1,164 @@
+import { describe, expect, it } from "vitest";
+import {
+  MIN_PREVIEW_H,
+  MIN_PREVIEW_W,
+  MIN_TIMELINE_H,
+  RAIL_W,
+  defaultPanelWidths,
+  fitPanelWidths,
+  fitTimelineHeight,
+} from "./fitPanels";
+
+function fitAt(viewportWidth: number) {
+  return fitPanelWidths(viewportWidth, defaultPanelWidths(viewportWidth));
+}
+
+describe("defaultPanelWidths", () => {
+  // Frozen literals, deliberately NOT compared against the live hook: the hook
+  // now delegates here, so asserting against it would be circular and would
+  // silently stop guarding the "wide windows are untouched" promise.
+  it.each([
+    [1680, 384, 424],
+    [1512, 384, 424],
+    [1280, 329, 364],
+  ])("keeps %ipx identical to the pre-change defaults", (vw, left, right) => {
+    expect(defaultPanelWidths(vw)).toEqual({ left, right });
+  });
+});
+
+describe("fitPanelWidths", () => {
+  it.each([1680, 1512, 1280])("leaves %ipx untouched by reconciliation", (vw) => {
+    const preferred = defaultPanelWidths(vw);
+    const fitted = fitPanelWidths(vw, preferred);
+    expect(fitted.left).toBe(preferred.left);
+    expect(fitted.right).toBe(preferred.right);
+    expect(fitted.autoCollapseLeft).toBe(false);
+    expect(fitted.autoCollapseRight).toBe(false);
+  });
+
+  it.each([1680, 1512, 1280, 1100, 960, 860, 760, 640, 560, 480])(
+    "never starves the preview at %ipx",
+    (vw) => {
+      expect(fitAt(vw).preview).toBeGreaterThanOrEqual(MIN_PREVIEW_W);
+    },
+  );
+
+  it("matches the projected preview widths from the plan", () => {
+    const projected: Array<[number, number]> = [
+      [1680, 866],
+      [1512, 698],
+      [1280, 581],
+      [1100, 499],
+      [960, 427],
+      [860, 360],
+      [760, 432],
+      [640, 592],
+      [560, 512],
+      [480, 432],
+    ];
+    for (const [vw, preview] of projected) {
+      expect({ vw, preview: fitAt(vw).preview }).toEqual({ vw, preview });
+    }
+  });
+
+  it("fits everything at its minimum at exactly 860, the derived threshold", () => {
+    const fitted = fitAt(860);
+    expect(fitted).toMatchObject({
+      left: 214,
+      right: 280,
+      preview: MIN_PREVIEW_W,
+      autoCollapseLeft: false,
+    });
+  });
+
+  it("rails the sidebar one pixel below the threshold", () => {
+    expect(fitAt(860).autoCollapseLeft).toBe(false);
+    expect(fitAt(859).autoCollapseLeft).toBe(true);
+    expect(fitAt(859).left).toBe(RAIL_W);
+  });
+
+  it("hides the inspector one pixel below its own threshold", () => {
+    expect(fitAt(700).autoCollapseRight).toBe(false);
+    expect(fitAt(700).right).toBeGreaterThan(0);
+    expect(fitAt(699).autoCollapseRight).toBe(true);
+    expect(fitAt(699).right).toBe(0);
+  });
+
+  it("never drives an EXPANDED sidebar below its usable minimum", () => {
+    // Guards the leftFloor/rightFloor split: the squeeze fallback may only reach
+    // the rail width when the panel is actually railed.
+    const fitted = fitPanelWidths(900, { left: 600, right: 600 });
+    expect(fitted.left).toBeGreaterThanOrEqual(200);
+    expect(fitted.right).toBeGreaterThanOrEqual(280);
+  });
+
+  it("caps a single panel at 40% of the window", () => {
+    expect(fitPanelWidths(1600, { left: 900, right: 280 }).left).toBe(640);
+  });
+
+  it("lets the preview floor win when the cap alone is not enough", () => {
+    // 40% of 1000 is 400, but 400 + 280 + 6 leaves the preview at 314.
+    const fitted = fitPanelWidths(1000, { left: 900, right: 280 });
+    expect(fitted.left).toBe(354);
+    expect(fitted.preview).toBe(MIN_PREVIEW_W);
+  });
+
+  it("survives a viewport narrower than the preview floor alone", () => {
+    const fitted = fitAt(300);
+    expect(fitted.preview).toBeGreaterThanOrEqual(0);
+    expect(Number.isNaN(fitted.preview)).toBe(false);
+    expect(fitted.left).toBe(RAIL_W);
+  });
+
+  it("returns preferences untouched before the shell is measured", () => {
+    const preferred = { left: 320, right: 400 };
+    expect(fitPanelWidths(0, preferred)).toMatchObject({ ...preferred, preview: 0 });
+    expect(fitPanelWidths(Number.NaN, preferred)).toMatchObject(preferred);
+  });
+
+  it("is idempotent, so a resize loop cannot oscillate", () => {
+    for (const vw of [1512, 1100, 860, 760, 560]) {
+      const once = fitPanelWidths(vw, defaultPanelWidths(vw));
+      const twice = fitPanelWidths(vw, once);
+      expect(twice).toEqual(once);
+    }
+  });
+});
+
+describe("fitTimelineHeight", () => {
+  it("keeps a height that fits", () => {
+    expect(fitTimelineHeight(717, 429)).toBe(429);
+  });
+
+  it("reclaims height for the preview when the shell shrinks", () => {
+    // The measured bug: mounted at 760 tall, dragged to 520. Preview was 47px.
+    expect(fitTimelineHeight(477, 429)).toBe(477 - MIN_PREVIEW_H);
+    expect(477 - fitTimelineHeight(477, 429)).toBeGreaterThanOrEqual(MIN_PREVIEW_H);
+  });
+
+  it("raises a too-small preference to the timeline minimum", () => {
+    expect(fitTimelineHeight(717, 10)).toBe(MIN_TIMELINE_H);
+  });
+
+  it("keeps the timeline usable when the shell is shorter than both minimums", () => {
+    expect(fitTimelineHeight(120, 429)).toBe(MIN_TIMELINE_H);
+  });
+
+  it("returns the preference before the shell is measured", () => {
+    expect(fitTimelineHeight(0, 429)).toBe(429);
+  });
+
+  it("is idempotent", () => {
+    const once = fitTimelineHeight(477, 429);
+    expect(fitTimelineHeight(477, once)).toBe(once);
+  });
+
+  it("would return the preferred height if callers kept one", () => {
+    // The helper is preference-preserving; the vertical CALLER still stores the
+    // clamped value, so a shrink-then-grow does not restore the old height the
+    // way panel widths do. Tracked as a known asymmetry, not fixed here.
+    const preferred = 429;
+    expect(fitTimelineHeight(477, preferred)).toBe(277);
+    expect(fitTimelineHeight(717, preferred)).toBe(429);
+  });
+});
diff --git a/packages/studio/src/utils/fitPanels.ts b/packages/studio/src/utils/fitPanels.ts
new file mode 100644
index 000000000..f3d584cc1
--- /dev/null
+++ b/packages/studio/src/utils/fitPanels.ts
@@ -0,0 +1,151 @@
+/**
+ * Panel reconciliation for the Studio shell.
+ *
+ * The shell is `[sidebar | preview | inspector]` over a full-width timeline.
+ * Only the preview is flexible (`flex-1 min-w-0`), so without an explicit floor
+ * it absorbs every squeeze: at a 560px window the old 240/320 panel floors alone
+ * overflowed the viewport and the preview rendered at 2px, on a *fresh* load.
+ * The same held vertically once the window was resized after mount.
+ *
+ * Every caller (mount, resize, drag, keyboard nudge, preference restore) routes
+ * through {@link fitPanelWidths} / {@link fitTimelineHeight} so exactly one place
+ * decides who yields. The rule is: the preview gets its floor first, panels
+ * shrink toward their minimums, then collapse to rails.
+ */
+
+/** Smallest preview box worth editing in. Chosen, not derived — see the plan. */
+export const MIN_PREVIEW_W = 360;
+export const MIN_PREVIEW_H = 200;
+
+const MIN_LEFT = 200;
+const MIN_RIGHT = 280;
+/**
+ * Collapsed sidebar rail, as a layout FOOTPRINT: the `w-10` box (40) plus the
+ * `mr-0.5` gap (2) it contributes to the flex row. Measured in the browser at a
+ * 760px window: rail box 40, margin-right 2, preview starts at x=43.
+ */
+export const RAIL_W = 42;
+export const MIN_TIMELINE_H = 100;
+
+/** The two 3px resize seams between the three top-row panels. */
+const PANEL_SEAM_W = 6;
+
+/** No single panel may claim more than this share of the window. */
+const PANEL_MAX_RATIO = 0.4;
+
+/*
+ * Thresholds below are DERIVED from the minimums above, not chosen:
+ *
+ *   MIN_LEFT 200 + MIN_RIGHT 280 + MIN_PREVIEW_W 360 + PANEL_SEAM_W 6 = 846
+ *     -> below ~860 the sidebar can no longer fit at its minimum.
+ *   RAIL_W  40 + MIN_RIGHT 280 + MIN_PREVIEW_W 360 + PANEL_SEAM_W 6 = 686
+ *     -> below ~700 the inspector can no longer fit either.
+ *
+ * Change a minimum and move the matching threshold with it.
+ */
+const RAIL_LEFT_BELOW = 860;
+const RAIL_RIGHT_BELOW = 700;
+
+/**
+ * True when the window is narrow enough to force at least one panel to a rail.
+ * The sidebar threshold is the wider of the two, so it is the whole condition.
+ */
+export function railsEngaged(viewportWidth: number): boolean {
+  return viewportWidth < RAIL_LEFT_BELOW;
+}
+
+export interface PanelWidths {
+  left: number;
+  right: number;
+}
+
+export interface FittedPanelWidths extends PanelWidths {
+  preview: number;
+  /**
+   * Width-driven collapse. Derived per render; NEVER persisted and never written
+   * back to the user's own collapse preference — `leftCollapsed` lives in
+   * localStorage and `rightCollapsed` is synced into the shareable Studio URL,
+   * so a ten-second window drag must not rewrite either.
+   */
+  autoCollapseLeft: boolean;
+  autoCollapseRight: boolean;
+}
+
+function clamp(value: number, min: number, max: number): number {
+  return Math.max(min, Math.min(max, value));
+}
+
+/**
+ * The width each panel wants on a given viewport, before reconciliation.
+ * Upper bounds (384 / 424) are unchanged from the original implementation, which
+ * is what keeps windows at or above 1280px byte-identical to before.
+ */
+export function defaultPanelWidths(viewportWidth: number): PanelWidths {
+  return {
+    left: Math.max(MIN_LEFT, Math.min(384, Math.round(viewportWidth * 0.257))),
+    right: Math.max(MIN_RIGHT, Math.min(424, Math.round(viewportWidth * 0.284))),
+  };
+}
+
+/**
+ * Resolve preferred panel widths against the viewport, reserving the preview.
+ *
+ * Order of yielding: window-relative cap, then the inspector shrinks toward its
+ * minimum, then the sidebar, then rails. A viewport of 0 (element not measured
+ * yet) returns the preferences untouched so a pre-measurement frame cannot
+ * clobber a real preference.
+ */
+// fallow-ignore-next-line complexity
+export function fitPanelWidths(
+  viewportWidth: number,
+  preferred: PanelWidths,
+  /**
+   * Sides the user explicitly reopened while the window was narrow. A side
+   * listed here keeps its real width instead of the rail: without this the
+   * panel would render expanded at 42px and squash its own content.
+   */
+  reopened: { left: boolean; right: boolean } = { left: false, right: false },
+): FittedPanelWidths {
+  const vw = Number.isFinite(viewportWidth) ? Math.max(0, viewportWidth) : 0;
+  if (vw <= 0) {
+    return { ...preferred, preview: 0, autoCollapseLeft: false, autoCollapseRight: false };
+  }
+
+  const autoCollapseLeft = vw < RAIL_LEFT_BELOW && !reopened.left;
+  const autoCollapseRight = vw < RAIL_RIGHT_BELOW && !reopened.right;
+  const cap = Math.floor(vw * PANEL_MAX_RATIO);
+
+  // A railed panel's floor is its rail width; an open panel's floor is its
+  // minimum usable width. Without this split the squeeze fallback below would
+  // drive an *expanded* sidebar down to 40px and render its content unusable.
+  const leftFloor = autoCollapseLeft ? RAIL_W : MIN_LEFT;
+  const rightFloor = autoCollapseRight ? 0 : MIN_RIGHT;
+
+  let left = autoCollapseLeft ? RAIL_W : clamp(preferred.left, MIN_LEFT, Math.max(MIN_LEFT, cap));
+  let right = autoCollapseRight ? 0 : clamp(preferred.right, MIN_RIGHT, Math.max(MIN_RIGHT, cap));
+
+  const budget = vw - MIN_PREVIEW_W - PANEL_SEAM_W;
+  if (left + right > budget) right = Math.max(rightFloor, budget - left);
+  if (left + right > budget) left = Math.max(leftFloor, budget - right);
+
+  return {
+    left,
+    right,
+    preview: Math.max(0, vw - left - right - PANEL_SEAM_W),
+    autoCollapseLeft,
+    autoCollapseRight,
+  };
+}
+
+/**
+ * Resolve a preferred timeline height against the shell's measured height.
+ *
+ * Subsumes the clamping that used to be inlined in TimelineResizeDivider's
+ * pointer and keyboard handlers and duplicated (mount-only) in NLEContext.
+ */
+export function fitTimelineHeight(containerHeight: number, preferred: number): number {
+  const h = Number.isFinite(containerHeight) ? containerHeight : 0;
+  const wanted = Number.isFinite(preferred) ? preferred : MIN_TIMELINE_H;
+  if (h <= 0) return Math.max(MIN_TIMELINE_H, wanted);
+  return clamp(wanted, MIN_TIMELINE_H, Math.max(MIN_TIMELINE_H, h - MIN_PREVIEW_H));
+}