fix(studio): reveal the shown tab only when its strip narrows

A strip the user scrolled by hand no longer snaps back when another panel
changes. The load smoke measures the focused group's strip directly.

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

@@ -177,10 +177,25 @@ describe("dock tab fill", () => {
     const strip = stripOf("design");
     if (!strip) throw new Error("no design strip");
     await activate("variables");
-    // The four tabs fit, so the strip is unscrolled.
+    // The four tabs fit, so the strip is unscrolled; then it narrows to two and a half tabs.
     strip.scrollLeft = 0;
+    Object.defineProperty(strip, "clientWidth", { value: 1000, configurable: true });
+    act(() => resizeAll());
     Object.defineProperty(strip, "clientWidth", { value: 250, configurable: true });
     act(() => resizeAll());
     expect(strip.scrollLeft).toBe(3 * TAB_STEP + TAB_WIDTH + 1 - 250);
   });
+
+  it("leaves a strip the user scrolled alone when something else in the dock changes", async () => {
+    const strip = stripOf("design");
+    if (!strip) throw new Error("no design strip");
+    await activate("variables");
+    Object.defineProperty(strip, "clientWidth", { value: 250, configurable: true });
+    act(() => resizeAll());
+    // Scrolled back by hand to look at the first tabs, then another group takes focus.
+    strip.scrollLeft = 0;
+    await activate("assets");
+    act(() => resizeAll());
+    expect(strip.scrollLeft).toBe(0);
+  });
 });

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

@@ -22,13 +22,16 @@ function revealedScroll(box: { left: number; width: number }, scrollLeft: number
   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 {
+/**
+ * Reads where the shown tab sits; the write happens after every strip is read, so layout runs once.
+ * Only a strip that got narrower is scrolled, so a strip the user scrolled by hand stays put.
+ */
+function measureFill(list: HTMLElement, narrowed: boolean): () => 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;
+  const target = box && narrowed ? revealedScroll(box, scrollLeft, list.clientWidth) : scrollLeft;
   return () => {
     fill.hidden = !box;
     if (!box) return;
@@ -70,9 +73,16 @@ export function installTabFill(api: DockviewApi, root: HTMLElement): () => void
       observed.add(element);
     }
   }
+  const widths = new WeakMap<HTMLElement, number>();
+  function narrowed(list: HTMLElement) {
+    const width = list.clientWidth;
+    const before = widths.get(list) ?? width;
+    widths.set(list, width);
+    return width < before;
+  }
   function placeAll() {
     const strips = [...lists()];
-    const writes = strips.map(measureFill);
+    const writes = strips.map((list) => measureFill(list, narrowed(list)));
     for (const write of writes) write();
     for (const list of strips) markClippedEdges(list);
     observe(strips.flatMap((list) => [list, ...list.querySelectorAll(":scope > .dv-tab")]));

scripts/studio-runtime-smoke.mjs

@@ -101,9 +101,10 @@ export function studioSmokeApiResponse(method, requestUrl) {
 // about 10px narrower than macOS does; keep that much room so a strip that fits here fits on a Mac.
 const MAC_FONT_ALLOWANCE_PX = 10;
 
-/** Runs in the page: the strip's tabs, plus the allowance, must fit before its action slot. */
-function clippedStrip(index, allowance) {
-  const strip = document.querySelectorAll(".dv-tabs-container")[index];
+/** Runs in the page: the active group's tabs, plus the allowance, must fit before its actions. */
+function clippedStrip(allowance) {
+  const strip = document.querySelector(".dv-groupview.dv-active-group .dv-tabs-container");
+  if (!strip) return "no active dock strip";
   const actions = strip
     .closest(".dv-tabs-and-actions-container")
     ?.querySelector(".dv-right-actions-container");
@@ -125,13 +126,13 @@ async function dockStripErrors(page) {
     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()) {
+  for (const tab of shownTabs) {
     // 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);
+    const clipped = await page.evaluate(clippedStrip, 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}`,