fix(editor): restore an evicted tab's exact scroll offset (#2324)

src/editor/core/model/document.test.ts

@@ -347,6 +347,28 @@ describe("DocumentManager + slots", () => {
     for (const slot of slots) slot.dispose();
     manager.stop();
   });
+
+  it("restores a destroyed kept view at its exact scroll offset, not its top line", async () => {
+    const { manager } = setup({ "/p/a.txt": "line\n".repeat(500) });
+    const model = await manager.acquire("/p/a.txt");
+    const slot = new EditorSlot(model, "t0", host);
+    const view = slot.mount(document.body);
+    Object.defineProperty(view.scrollDOM, "scrollTop", { value: 137, configurable: true });
+    view.scrollDOM.dispatchEvent(new Event("scroll"));
+    slot.unmount();
+    const expected = view.scrollSnapshot().value as { range: unknown; yMargin: number; isSnapshot: boolean };
+    const others = Array.from({ length: 8 }, (_, i) => new EditorSlot(model, `t${i + 1}`, host));
+    for (const other of others) {
+      other.mount(document.body);
+      other.unmount();
+    }
+    const restored = (slot as unknown as { scroll: { value: typeof expected } }).scroll.value;
+    expect(restored.isSnapshot).toBe(true);
+    expect(restored.range).toEqual(expected.range);
+    expect(restored.yMargin).toBe(expected.yMargin);
+    for (const s of [slot, ...others]) s.dispose();
+    manager.stop();
+  });
 });
 
 describe("save cleanup", () => {

src/editor/core/model/slot.ts

@@ -242,9 +242,8 @@ export class EditorSlot {
     this.kept = null;
     const i = keptSlots.indexOf(this);
     if (i >= 0) keptSlots.splice(i, 1);
-    const top = topLineAt(view, this.scrollPos.top);
-    this.savedTopLine = top;
-    this.scroll = EditorView.scrollIntoView(view.state.doc.line(top + 1).from, { y: "start" });
+    this.savedTopLine = topLineAt(view, this.scrollPos.top);
+    this.scroll = scrollSnapshotAt(view, this.scrollPos);
     this.stored = view.state;
     view.destroy();
   }
@@ -399,6 +398,13 @@ function topLineOf(view: EditorView): number {
   return topLineAt(view, view.scrollDOM.scrollTop);
 }
 
+/** `view.scrollSnapshot()` at a tracked offset: a detached view's scrollDOM reports 0. */
+function scrollSnapshotAt(view: EditorView, pos: { top: number; left: number }): StateEffect<unknown> {
+  const atPos = Object.create(view) as EditorView;
+  Object.defineProperty(atPos, "scrollDOM", { value: { scrollTop: pos.top, scrollLeft: pos.left } });
+  return atPos.scrollSnapshot();
+}
+
 function topLineAt(view: EditorView, scrollTop: number): number {
   const block = view.lineBlockAtHeight(Math.max(0, scrollTop));
   return view.state.doc.lineAt(block.from).number - 1;