fix(app): preserve framed row paint

Author: Hona

packages/app/e2e/regression/session-timeline-shell-outline.spec.ts

@@ -3,6 +3,7 @@ import {
   assistantMessage,
   setupTimeline,
   shell,
+  textPart,
   toolPart,
   userMessage,
 } from "../performance/timeline-stability/fixture"
@@ -119,6 +120,44 @@ test("keeps the patch card inside a fractionally short virtual row", async ({ pa
   expect(geometry.clipMargin).toBe("0.5px")
 })
 
+test("allows paint rounding for every framed row but not fixed turn gaps", async ({ page }) => {
+  const secondUserID = "msg_outline_second_user"
+  await setupTimeline(page, {
+    messages: [
+      userMessage(undefined, {
+        summary: {
+          diffs: [
+            {
+              file: "src/summary.ts",
+              additions: 1,
+              deletions: 1,
+              patch: "@@ -1 +1 @@\n-export const value = 1\n+export const value = 2",
+            },
+          ],
+        },
+      }),
+      assistantMessage([textPart("prt_outline_text", "Assistant text")]),
+      userMessage(undefined, { id: secondUserID, created: 1700000010000 }),
+      assistantMessage([], {
+        id: "msg_outline_second_assistant",
+        parentID: secondUserID,
+        created: 1700000011000,
+      }),
+    ],
+  })
+  await expect(page.locator('[data-timeline-row="DiffSummary"]')).toBeVisible()
+  await expect(page.locator('[data-timeline-row="TurnGap"]')).toBeVisible()
+
+  const rows = await page.locator("[data-timeline-key]").evaluateAll((elements) =>
+    elements.map((element) => ({
+      tag: element.querySelector<HTMLElement>("[data-timeline-row]")?.dataset.timelineRow,
+      clipMargin: getComputedStyle(element).overflowClipMargin,
+    })),
+  )
+  expect(rows.filter((row) => row.tag !== "TurnGap").every((row) => row.clipMargin === "0.5px")).toBe(true)
+  expect(rows.filter((row) => row.tag === "TurnGap")).toEqual([{ tag: "TurnGap", clipMargin: "0px" }])
+})
+
 async function captureCardEdges(page: Page, card: Locator) {
   const box = await card.boundingBox()
   if (!box) throw new Error("Tool card bounds are unavailable")

packages/app/src/pages/session/timeline/message-timeline.tsx

@@ -1279,8 +1279,8 @@ export function MessageTimeline(props: {
           width: "100%",
           height: `${item().size}px`,
           overflow: "clip",
-          // Rounded virtual measurements can otherwise clip a tool card's outer border pixel.
-          "overflow-clip-margin": tool() ? "0.5px" : undefined,
+          // Rounded virtual measurements can otherwise clip a framed row's outer paint.
+          "overflow-clip-margin": row()._tag === "TurnGap" ? undefined : "0.5px",
         }}
       >
         <div