feat(app): make session timelines much faster AND without flicker or scroll jumps (#32331)

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

@@ -0,0 +1,46 @@
+import { expect, test } from "bun:test"
+import { createVirtualizer } from "@tanstack/solid-virtual"
+import { createRoot, createSignal } from "solid-js"
+
+test("reactive count updates preserve measured row sizes", () => {
+  createRoot((dispose) => {
+    const [count, setCount] = createSignal(2)
+    const virtualizer = createVirtualizer<HTMLDivElement, HTMLDivElement>({
+      get count() {
+        return count()
+      },
+      getScrollElement: () => null,
+      estimateSize: () => 60,
+      initialRect: { width: 800, height: 600 },
+    })
+
+    expect(virtualizer.getTotalSize()).toBe(120)
+    virtualizer.resizeItem(0, 100)
+    expect(virtualizer.getTotalSize()).toBe(160)
+
+    setCount(3)
+
+    expect(virtualizer.itemSizeCache.get(0)).toBe(100)
+    expect(virtualizer.getTotalSize()).toBe(220)
+    dispose()
+  })
+})
+
+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()
+  })
+})