fix(studio): keep a hand-scrolled strip put through a maximise

A hidden strip keeps its scroll, so a strip is compared with its last visible
shape and a strip that comes back unchanged is left where the user put it.

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

@@ -199,16 +199,43 @@ describe("dock tab fill", () => {
     expect(strip.scrollLeft).toBe(0);
   });
 
-  it("reveals the shown tab again when its strip comes back from hidden", async () => {
+  it("keeps a hand scroll through another group being maximised and restored", 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());
-    // Another group is maximised, hiding this strip; the browser drops a hidden strip's scroll.
+    strip.scrollLeft = 0;
+    // Hidden while another group is maximised; the browser keeps a hidden strip's scroll.
     Object.defineProperty(strip, "clientWidth", { value: 0, configurable: true });
     act(() => resizeAll());
+    Object.defineProperty(strip, "clientWidth", { value: 250, configurable: true });
+    act(() => resizeAll());
+    expect(strip.scrollLeft).toBe(0);
+  });
+
+  it("leaves a hand scroll alone when its strip widens", async () => {
+    const strip = stripOf("design");
+    if (!strip) throw new Error("no design strip");
+    await activate("variables");
+    Object.defineProperty(strip, "clientWidth", { value: 150, configurable: true });
+    act(() => resizeAll());
     strip.scrollLeft = 0;
+    // Wider, but the shown tab is still out of view where the user left the strip.
+    Object.defineProperty(strip, "clientWidth", { value: 200, configurable: true });
+    act(() => resizeAll());
+    expect(strip.scrollLeft).toBe(0);
+  });
+
+  it("reveals the shown tab when its strip comes back narrower than it left", async () => {
+    const strip = stripOf("design");
+    if (!strip) throw new Error("no design strip");
+    await activate("variables");
+    strip.scrollLeft = 0;
+    Object.defineProperty(strip, "clientWidth", { value: 1000, configurable: true });
+    act(() => resizeAll());
+    Object.defineProperty(strip, "clientWidth", { value: 0, 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);

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

@@ -19,12 +19,12 @@ function fillOf(list: HTMLElement): HTMLElement {
 type StripShape = { width: number; tab: HTMLElement | null; left: number; size: number };
 
 /**
- * True when the strip narrowed, came back from hidden, or its shown tab moved or resized. dockview
+ * True when the strip narrowed or its shown tab moved or resized since it was last visible. dockview
  * reveals a tab only when it is activated, and the user's own scrolling changes none of these.
  */
 function reshaped(before: StripShape | undefined, now: StripShape) {
   if (!before || now.width === 0) return false;
-  if (now.width < before.width || before.width === 0) return true;
+  if (now.width < before.width) return true;
   return now.tab !== before.tab || now.left !== before.left || now.size !== before.size;
 }
 
@@ -46,7 +46,8 @@ function measureFill(list: HTMLElement, shapes: WeakMap<HTMLElement, StripShape>
     size: tab?.offsetWidth ?? 0,
   };
   const reveal = tab !== null && reshaped(shapes.get(list), shape);
-  shapes.set(list, shape);
+  // A hidden strip (another group maximised) keeps its scroll, so it is compared with its last visible shape.
+  if (shape.width > 0) shapes.set(list, shape);
   const scrollLeft = list.scrollLeft;
   const target = reveal ? revealedScroll(shape, scrollLeft) : scrollLeft;
   return () => {