chore: refactoring and tests, splitting up files (#12495)

packages/app/src/pages/session/file-tab-scroll.test.ts

@@ -0,0 +1,40 @@
+import { describe, expect, test } from "bun:test"
+import { nextTabListScrollLeft } from "./file-tab-scroll"
+
+describe("nextTabListScrollLeft", () => {
+  test("does not scroll when width shrinks", () => {
+    const left = nextTabListScrollLeft({
+      prevScrollWidth: 500,
+      scrollWidth: 420,
+      clientWidth: 300,
+      prevContextOpen: false,
+      contextOpen: false,
+    })
+
+    expect(left).toBeUndefined()
+  })
+
+  test("scrolls to start when context tab opens", () => {
+    const left = nextTabListScrollLeft({
+      prevScrollWidth: 400,
+      scrollWidth: 500,
+      clientWidth: 320,
+      prevContextOpen: false,
+      contextOpen: true,
+    })
+
+    expect(left).toBe(0)
+  })
+
+  test("scrolls to right edge for new file tabs", () => {
+    const left = nextTabListScrollLeft({
+      prevScrollWidth: 500,
+      scrollWidth: 780,
+      clientWidth: 300,
+      prevContextOpen: true,
+      contextOpen: true,
+    })
+
+    expect(left).toBe(480)
+  })
+})