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

packages/ui/src/components/scroll-view.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test } from "bun:test"
-import { scrollKey } from "./scroll-view"
+import { scrollKey, scrollTopFromThumbPointer } from "./scroll-view"
 
 describe("scrollKey", () => {
   test("maps plain navigation keys", () => {
@@ -17,3 +17,38 @@ describe("scrollKey", () => {
     expect(scrollKey({ key: "End", altKey: false, ctrlKey: false, metaKey: false, shiftKey: true })).toBeUndefined()
   })
 })
+
+describe("scrollTopFromThumbPointer", () => {
+  test("keeps downward thumb movement monotonic when content height changes", () => {
+    const first = scrollTopFromThumbPointer({
+      pointer: 300,
+      viewportTop: 100,
+      grabOffset: 12,
+      clientHeight: 600,
+      scrollHeight: 6_000,
+      thumbHeight: 60,
+    })
+    const second = scrollTopFromThumbPointer({
+      pointer: 320,
+      viewportTop: 100,
+      grabOffset: 12,
+      clientHeight: 600,
+      scrollHeight: 60_000,
+      thumbHeight: 32,
+    })
+
+    expect(second).toBeGreaterThan(first)
+  })
+
+  test("clamps pointer positions to the scroll range", () => {
+    const input = {
+      viewportTop: 100,
+      grabOffset: 12,
+      clientHeight: 600,
+      scrollHeight: 6_000,
+      thumbHeight: 60,
+    }
+    expect(scrollTopFromThumbPointer({ ...input, pointer: 0 })).toBe(0)
+    expect(scrollTopFromThumbPointer({ ...input, pointer: 1_000 })).toBe(5_400)
+  })
+})