test(e2e): poll for visible Monaco diff line before clicking

clickVisibleDiffLine read Monaco's virtualized .view-line set in a single
evaluate right after a tab switch, but Monaco re-lays-out its diff lines
asynchronously. On a contended CI shard the visible set is briefly empty, so
the evaluate threw 'visible combined diff line not found' before Monaco
painted. Poll until a line is in the viewport instead of failing on first miss.

Co-authored-by: Orca <help@stably.ai>

tests/e2e/combined-diff-scroll-restore.spec.ts

@@ -329,34 +329,50 @@ function getLargestBackwardScrollJump(samples: readonly ScrollProbeSample[]): nu
 }
 
 async function clickVisibleDiffLine(page: Page): Promise<void> {
-  const linePoint = await page.evaluate(() => {
-    const container = document.querySelector<HTMLElement>('.combined-diff-scroll-container')
-    if (!container) {
-      throw new Error('combined diff scroll container not found')
-    }
-    const containerRect = container.getBoundingClientRect()
-    const visibleLine = Array.from(
-      container.querySelectorAll<HTMLElement>('.monaco-diff-editor .view-line')
-    ).find((line) => {
-      const rect = line.getBoundingClientRect()
-      return (
-        rect.height > 0 &&
-        rect.bottom > containerRect.top &&
-        rect.top < containerRect.bottom &&
-        rect.right > containerRect.left &&
-        rect.left < containerRect.right
-      )
-    })
-    if (!visibleLine) {
-      throw new Error('visible combined diff line not found')
-    }
-    const rect = visibleLine.getBoundingClientRect()
-    return {
-      x: rect.left + Math.min(12, Math.max(1, rect.width / 2)),
-      y: rect.top + rect.height / 2
-    }
-  })
+  // Why: after a tab switch Monaco re-lays-out its virtualized diff lines
+  // asynchronously, so the visible .view-line set is briefly empty on a loaded
+  // CI runner. Poll until a line is painted in the viewport instead of reading
+  // it once and throwing on the first miss.
+  let linePoint: { x: number; y: number } | null = null
+  await expect
+    .poll(
+      async () => {
+        linePoint = await page.evaluate(() => {
+          const container = document.querySelector<HTMLElement>('.combined-diff-scroll-container')
+          if (!container) {
+            return null
+          }
+          const containerRect = container.getBoundingClientRect()
+          const visibleLine = Array.from(
+            container.querySelectorAll<HTMLElement>('.monaco-diff-editor .view-line')
+          ).find((line) => {
+            const rect = line.getBoundingClientRect()
+            return (
+              rect.height > 0 &&
+              rect.bottom > containerRect.top &&
+              rect.top < containerRect.bottom &&
+              rect.right > containerRect.left &&
+              rect.left < containerRect.right
+            )
+          })
+          if (!visibleLine) {
+            return null
+          }
+          const rect = visibleLine.getBoundingClientRect()
+          return {
+            x: rect.left + Math.min(12, Math.max(1, rect.width / 2)),
+            y: rect.top + rect.height / 2
+          }
+        })
+        return linePoint !== null
+      },
+      { timeout: 10_000, message: 'visible combined diff line not found' }
+    )
+    .toBe(true)
 
+  if (!linePoint) {
+    throw new Error('visible combined diff line not found')
+  }
   await page.mouse.click(linePoint.x, linePoint.y)
 }