fix(app): preserve timeline bottom anchoring (#36160)

packages/app/test-browser/solid-virtual.test.ts

@@ -1,8 +1,32 @@
 import { expect, test } from "bun:test"
-import { createVirtualizer, defaultRangeExtractor } from "@tanstack/solid-virtual"
+import { createVirtualizer, defaultRangeExtractor, Virtualizer } from "@tanstack/solid-virtual"
 import { createRoot, createSignal } from "solid-js"
 import { filterVirtualIndexes } from "@/pages/session/timeline/virtual-items"
 
+test("end anchoring survives consecutive resizes when the first scroll write is clamped", () => {
+  const writes: { offset: number; adjustments?: number }[] = []
+  const virtualizer = new Virtualizer<HTMLDivElement, HTMLDivElement>({
+    count: 5,
+    estimateSize: () => 50,
+    initialOffset: 50,
+    initialRect: { width: 400, height: 200 },
+    anchorTo: "end",
+    scrollEndThreshold: 1,
+    getScrollElement: () => null,
+    scrollToFn: (offset, options) => writes.push({ offset, adjustments: options.adjustments }),
+    observeElementRect: () => {},
+    observeElementOffset: () => {},
+  })
+
+  virtualizer.getTotalSize()
+  virtualizer.resizeItem(4, 120)
+  expect(writes).toEqual([{ offset: 50, adjustments: 70 }])
+  writes.length = 0
+
+  virtualizer.resizeItem(4, 200)
+  expect(writes).toEqual([{ offset: 120, adjustments: 80 }])
+})
+
 test("reactive count updates preserve measured row sizes", () => {
   createRoot((dispose) => {
     const [count, setCount] = createSignal(2)
@@ -42,23 +66,26 @@ test("initial rect projects rows before a scroll element connects", () => {
   })
 })
 
-test("logical scroll offset includes pending measurement adjustments", () => {
-  createRoot((dispose) => {
-    const virtualizer = createVirtualizer<HTMLDivElement, HTMLDivElement>({
-      count: 2,
-      getScrollElement: () => null,
-      estimateSize: () => 60,
-      initialOffset: 100,
-      initialRect: { width: 800, height: 60 },
-    })
-
-    virtualizer.getTotalSize()
-    virtualizer.resizeItem(0, 100)
-
-    expect(virtualizer.scrollOffset).toBe(100)
-    expect(virtualizer.getLogicalScrollOffset()).toBe(140)
-    dispose()
+test("clamps oversized offsets with scroll margin and padding changes", () => {
+  const options = (paddingEnd: number) => ({
+    count: 20,
+    estimateSize: () => 60,
+    initialOffset: Number.MAX_SAFE_INTEGER,
+    initialRect: { width: 800, height: 600 },
+    scrollMargin: 64,
+    paddingEnd,
+    overscan: 1,
+    getScrollElement: () => null,
+    scrollToFn: () => {},
+    observeElementRect: () => {},
+    observeElementOffset: () => {},
   })
+  const virtualizer = new Virtualizer<HTMLDivElement, HTMLDivElement>(options(64))
+
+  expect(virtualizer.getVirtualItems().map((item) => item.index)).toEqual([10, 11, 12, 13, 14, 15, 16, 17, 18, 19])
+
+  virtualizer.setOptions(options(600))
+  expect(virtualizer.getVirtualItems().map((item) => item.index)).toEqual([18, 19])
 })
 
 test("stale pinned indexes do not produce missing virtual items after count shrinks", () => {