diff --git a/ui-tui/src/__tests__/brandingRenderWidth.test.ts b/ui-tui/src/__tests__/brandingRenderWidth.test.ts
index 8166c0bf..8d8c52a0 100644
--- a/ui-tui/src/__tests__/brandingRenderWidth.test.ts
+++ b/ui-tui/src/__tests__/brandingRenderWidth.test.ts
@@ -63,18 +63,16 @@ const info = (over: Partial<SessionInfo> = {}): SessionInfo => ({
   ...over
 })
 
-const boxRows = (cols: number, sessionInfo: SessionInfo, theme: Theme = DEFAULT_THEME) =>
-  render(cols, transcriptPanelWidth(cols), sessionInfo, theme)
-
-/** Same wrapper, but the panel is told a width its container does not have. */
-const boxRowsWithMaxWidth = (cols: number, maxWidth: number) => render(cols, maxWidth, info(), DEFAULT_THEME)
-
-async function render(
+/** `oversizedContainer` drops the scrollbar gutter, reproducing the first paint
+ *  where the ScrollBox has not yet reserved it. */
+async function boxRows(
   cols: number,
-  maxWidth: number,
   sessionInfo: SessionInfo,
-  theme: Theme = DEFAULT_THEME
+  theme: Theme = DEFAULT_THEME,
+  { oversizedContainer = false }: { oversizedContainer?: boolean } = {}
 ): Promise<string[]> {
+  const maxWidth = transcriptPanelWidth(cols)
+  const gutter = oversizedContainer ? 0 : TRANSCRIPT_SCROLLBAR_GUTTER
   Object.defineProperty(process.stdout, 'columns', { configurable: true, value: cols, writable: true })
 
   const stdout = new PassThrough()
@@ -112,11 +110,7 @@ async function render(
           React.createElement(SessionPanel, { info: sessionInfo, maxWidth, sid: 'a1b2c3d4', t: theme })
         )
       ),
-      React.createElement(Box, {
-        flexShrink: 0,
-        marginLeft: TRANSCRIPT_SCROLLBAR_GUTTER - 1,
-        width: TRANSCRIPT_SCROLLBAR_GUTTER - 1
-      })
+      gutter ? React.createElement(Box, { flexShrink: 0, marginLeft: gutter - 1, width: gutter - 1 }) : null
     ),
     {
       patchConsole: false,
@@ -181,22 +175,29 @@ describe('header box render width', () => {
     }
   })
 
-  // The shipped regression: appLayout passed `composer.cols - 2` (the COMPOSER's
-  // reserve) instead of the transcript's, so the panel believed it had two more
-  // columns than its container had. Combined with an explicit `width={cols}` on
-  // the box, the right border was pushed off screen — the box rendered with a
-  // left edge, a top rule running to the terminal edge, and no `╮`/`│`/`╯`.
+  // The regression that made the border invisible in a real terminal.
+  //
+  // On the first paint the transcript's ScrollBox has not settled and reports
+  // its full width, WITHOUT the scrollbar gutter its steady-state layout
+  // reserves. A box that sizes to its container therefore renders two columns
+  // too wide, gets clipped, and — because the intro row is committed to
+  // scrollback and never repainted — stays broken until a resize.
   //
-  // The panel must now survive a caller that over-reports: with no hard width it
-  // stretches to its real container regardless of what it was told.
-  it.each([34, 60, 100, 140])('ignores an over-reported maxWidth at %i columns', async cols => {
-    const rows = await boxRowsWithMaxWidth(cols, cols - 2)
+  // So the panel must hold its own width against a container that is bigger
+  // than the steady-state one. `oversizedContainer` reproduces frame 1 by
+  // omitting the gutter.
+  it.each(WIDTHS)('holds its width when the container is mis-measured at %i columns', async cols => {
+    const rows = await boxRows(cols, info(), DEFAULT_THEME, { oversizedContainer: true })
     const top = rows.find(r => r.startsWith('╭'))
+    const bottom = rows.find(r => r.startsWith('╰'))
 
     expect(rows.length).toBeGreaterThan(0)
     expect(top).toBeDefined()
+    expect(bottom).toBeDefined()
     expect(top?.endsWith('╮')).toBe(true)
+    expect(bottom?.endsWith('╯')).toBe(true)
 
+    // Still the steady-state width, not the container's inflated one.
     for (const row of rows) {
       expect(stringWidth(row)).toBe(transcriptPanelWidth(cols))
     }
diff --git a/ui-tui/src/components/branding.tsx b/ui-tui/src/components/branding.tsx
index 54296311..7d424a58 100644
--- a/ui-tui/src/components/branding.tsx
+++ b/ui-tui/src/components/branding.tsx
@@ -676,13 +676,23 @@ export function SessionPanel({ info, logoPalette, maxWidth, sid, t }: SessionPan
       marginBottom={1}
       paddingX={1}
       paddingY={1}
-      // Deliberately NO explicit width: the box stretches to its container, so
-      // the right border is wherever the container actually ends. Pinning it to
-      // `cols` instead makes the border a *prediction* of the container width,
-      // and when that prediction was two columns high (the transcript reserves a
-      // scrollbar gutter the composer budget doesn't) the right border was
-      // pushed off screen entirely. `cols` still drives the column split below,
-      // where being slightly off only shifts a truncation point.
+      // Explicit width, deliberately — do NOT let this box size to its
+      // container.
+      //
+      // On the FIRST paint the transcript's ScrollBox has not settled: it
+      // reports its full width, without the scrollbar gutter its steady-state
+      // layout reserves. A container-sized box therefore renders two columns
+      // wider than the terminal can show and its right border is clipped off
+      // screen, staying that way because the intro row is committed to
+      // scrollback and never repainted — until a resize forces a relayout, at
+      // which point the border appears. That is exactly the "I have to resize
+      // to see the border" symptom.
+      //
+      // `cols` does not have that problem: it comes from `maxWidth`
+      // (transcriptPanelWidth) and process.stdout.columns, both correct on the
+      // first frame. Measured on a real pty: cols=196 on frame 1 at a 200-col
+      // terminal, while the container claimed 198.
+      width={cols}
     >
       {wide && (
         <Box alignItems="center" flexDirection="column" width={leftW}>