fix(studio): keep the shown tab in view when its strip narrows

Focusing a panel draws its strip actions, which could hide the shown tab
of a strip that only just fit. The load smoke checks move into their own
function to keep the smoke runner simple.

packages/studio/src/components/dock/DockTabs.test.tsx

@@ -11,11 +11,23 @@ import { PANEL_IDS, type PanelId } from "./panelRegistry";
 
 (globalThis as unknown as { IS_REACT_ACT_ENVIRONMENT: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
 
+// happy-dom never resizes anything; a test calls `resizeAll` to stand in for a real resize.
+const liveObservers = new Set<ResizeObserverStub>();
 class ResizeObserverStub {
+  readonly callback: () => void;
+  constructor(callback: () => void) {
+    this.callback = callback;
+    liveObservers.add(this);
+  }
   observe() {}
   unobserve() {}
-  disconnect() {}
+  disconnect() {
+    liveObservers.delete(this);
+  }
 }
+const resizeAll = () => {
+  for (const observer of liveObservers) observer.callback();
+};
 (globalThis as unknown as { ResizeObserver: unknown }).ResizeObserver = ResizeObserverStub;
 
 // happy-dom has no layout: a tab sits at 100px per position and is 90px wide.
@@ -160,4 +172,15 @@ describe("dock tab fill", () => {
     expect(stripOf("design")?.querySelectorAll(".hf-dock-tab-fill")).toHaveLength(1);
     expect(fill()?.style.transform).toBe(`translateX(${2 * TAB_STEP}px)`);
   });
+
+  it("keeps the shown tab whole when its strip narrows, as when the strip's actions appear", async () => {
+    const strip = stripOf("design");
+    if (!strip) throw new Error("no design strip");
+    await activate("variables");
+    // The four tabs fit, so the strip is unscrolled.
+    strip.scrollLeft = 0;
+    Object.defineProperty(strip, "clientWidth", { value: 250, configurable: true });
+    act(() => resizeAll());
+    expect(strip.scrollLeft).toBe(3 * TAB_STEP + TAB_WIDTH + 1 - 250);
+  });
 });

packages/studio/src/components/dock/dockTabFill.ts

@@ -15,16 +15,26 @@ function fillOf(list: HTMLElement): HTMLElement {
   return fill;
 }
 
+/** The scroll that shows the whole shown tab, e.g. after the strip narrows because its actions appeared. */
+function revealedScroll(box: { left: number; width: number }, scrollLeft: number, width: number) {
+  if (box.left < scrollLeft) return box.left;
+  // Offset sizes are rounded while the tab's edge is fractional; a pixel over is clamped at the end.
+  return Math.max(scrollLeft, box.left + box.width + 1 - width);
+}
+
 /** Reads where the shown tab sits; the write happens after every strip is read, so layout runs once. */
 function measureFill(list: HTMLElement): () => void {
   const fill = fillOf(list);
   const tab = list.querySelector<HTMLElement>(":scope > .dv-active-tab");
   const box = tab ? { width: tab.offsetWidth, left: tab.offsetLeft } : null;
+  const scrollLeft = list.scrollLeft;
+  const target = box ? revealedScroll(box, scrollLeft, list.clientWidth) : scrollLeft;
   return () => {
     fill.hidden = !box;
     if (!box) return;
     fill.style.width = `${box.width}px`;
     fill.style.transform = `translateX(${box.left}px)`;
+    if (target !== scrollLeft) list.scrollLeft = target;
   };
 }
 
@@ -39,7 +49,8 @@ function markClippedEdges(list: HTMLElement) {
 }
 
 /**
- * Slides one fill per tab strip under its shown tab, and keeps the clipped-edge marks current.
+ * Slides one fill per tab strip under its shown tab, keeps that tab in view, and keeps the
+ * clipped-edge marks current.
  * Returns a disposer.
  */
 export function installTabFill(api: DockviewApi, root: HTMLElement): () => void {

scripts/studio-runtime-smoke.mjs

@@ -114,6 +114,33 @@ function clippedStrip(index, allowance) {
   return `${labels.join(", ")}: ${strip.scrollWidth}px of tabs in ${Math.floor(room)}px`;
 }
 
+/** Every dock strip gives its tabs the whole width it can, and fits them while its group is active. */
+async function dockStripErrors(page) {
+  const found = [];
+  const reservedSlots = await page.$$eval(
+    ".dv-groupview.dv-inactive-group .dv-right-actions-container",
+    (slots) => slots.filter((slot) => slot.getBoundingClientRect().width > 0).length,
+  );
+  if (reservedSlots > 0) {
+    found.push(`${reservedSlots} inactive dock strips hold width for actions they do not draw`);
+  }
+  const shownTabs = await page.$$(".dv-tabs-container .dv-active-tab");
+  for (const [index, tab] of shownTabs.entries()) {
+    // An active group draws its strip actions, so this is the least room its tabs get.
+    await tab.click();
+    await page.evaluate(
+      () => new Promise((resolve) => requestAnimationFrame(() => requestAnimationFrame(resolve))),
+    );
+    const clipped = await page.evaluate(clippedStrip, index, MAC_FONT_ALLOWANCE_PX);
+    if (clipped) {
+      found.push(
+        `Dock tab strip clips a label at the default layout (${MAC_FONT_ALLOWANCE_PX}px macOS allowance): ${clipped}`,
+      );
+    }
+  }
+  return found;
+}
+
 export function isExpectedStudioSmokeError(message) {
   return message.includes("favicon.ico");
 }
@@ -162,27 +189,7 @@ export async function runStudioRuntimeSmoke(targetUrl) {
       return textContent.includes("Something went wrong") ? textContent : null;
     });
     if (errorBoundary) errors.push(`React error boundary triggered: ${errorBoundary}`);
-    const reservedSlots = await page.$$eval(
-      ".dv-groupview.dv-inactive-group .dv-right-actions-container",
-      (slots) => slots.filter((slot) => slot.getBoundingClientRect().width > 0).length,
-    );
-    if (reservedSlots > 0) {
-      errors.push(`${reservedSlots} inactive dock strips hold width for actions they do not draw`);
-    }
-    const shownTabs = await page.$$(".dv-tabs-container .dv-active-tab");
-    for (const [index, tab] of shownTabs.entries()) {
-      // An active group draws its strip actions, so this is the least room its tabs get.
-      await tab.click();
-      await page.evaluate(
-        () => new Promise((resolve) => requestAnimationFrame(() => requestAnimationFrame(resolve))),
-      );
-      const clipped = await page.evaluate(clippedStrip, index, MAC_FONT_ALLOWANCE_PX);
-      if (clipped) {
-        errors.push(
-          `Dock tab strip clips a label at the default layout (${MAC_FONT_ALLOWANCE_PX}px macOS allowance): ${clipped}`,
-        );
-      }
-    }
+    errors.push(...(await dockStripErrors(page)));
   } finally {
     await browser.close();
   }