fix(ui): contain idle caret paint so agent panes stop burning CPU (#10554)

An idle agent pane kept ~40% of a core busy just by being frontmost. The
xterm cursor and the native chat caret blink with no paint-containment
boundary, so Chromium treated each blink as damage to the whole pane
ancestry and re-rasterized it twice a second.

- `.xterm-container` and the native composer's input shell get
  `contain: paint`, bounding blink damage to the surface that blinks.
- The mention hint gains `z-20` to match the slash picker: a contained
  element becomes a stacking context and paints at z-index 0 in tree
  order, which would otherwise cover the hint's drop shadow.

Also records that DECSCUSR pins `decPrivateModes.cursorBlink`, which wins
over the option in `_updateCursorBlink` — so parking `cursorBlink` does not
reliably stop a hidden pane blinking. Pre-existing, documented only.

Co-authored-by: Wooseong Kim <innocarpe@users.noreply.github.com>

src/renderer/src/assets/terminal-container-geometry.test.ts

@@ -15,4 +15,8 @@ describe('terminal container geometry', () => {
       /\.pane-link-tooltip\s*{[^}]*height:\s*var\(--orca-terminal-link-tooltip-height\);/s
     )
   })
+
+  it('bounds cursor-blink repaints to the terminal surface (#10481)', () => {
+    expect(terminalCss).toMatch(/\.xterm-container\s*{[^}]*contain:\s*paint;/s)
+  })
 })

src/renderer/src/assets/terminal.css

@@ -512,6 +512,10 @@
   height: calc(100% - var(--pane-padding-y, 4px));
   margin-top: var(--pane-padding-y, 4px);
   margin-left: var(--pane-padding-x, 4px);
+  /* Why (#10481): a blinking cursor otherwise invalidates paint all the way up
+     the pane ancestry. The link tooltip and drag handle are .pane siblings, so
+     clipping to this box costs no visible chrome. */
+  contain: paint;
 }
 
 /* When a pane has a title, shift the terminal content down to make room.

src/renderer/src/components/native-chat/NativeChatAutocompleteMenus.tsx

@@ -276,7 +276,10 @@ export function NativeChatMentionHint({
         event.preventDefault()
         onAccept()
       }}
-      className="absolute bottom-full left-3 right-3 mb-1 flex w-auto items-center gap-2 rounded-md border border-border bg-popover px-3 py-1.5 text-left text-xs text-muted-foreground shadow-md sm:left-4 sm:right-4"
+      // Why z-20: matches the slash picker. The composer shell below is a paint
+      // containment boundary (#10481), so it now paints at z-index 0 in tree
+      // order and would otherwise cover this hint's drop shadow.
+      className="absolute bottom-full left-3 right-3 z-20 mb-1 flex w-auto items-center gap-2 rounded-md border border-border bg-popover px-3 py-1.5 text-left text-xs text-muted-foreground shadow-md sm:left-4 sm:right-4"
     >
       {translate('components.native-chat.composer.mentionHint', 'Referencing file:')}{' '}
       <span className="font-medium text-foreground">@{query || '…'}</span>

src/renderer/src/components/native-chat/NativeChatComposerField.tsx

@@ -192,7 +192,15 @@ export function NativeChatComposerField({
               // no focus/click border flash. The box is a container, not a
               // focus target.
               'rounded-lg border border-border p-1.5 shadow-xs',
-              'bg-muted/50 dark:bg-input/40'
+              'bg-muted/50 dark:bg-input/40',
+              // Why (#10481): the native caret blink invalidates paint up to the
+              // nearest containment boundary; without this the whole transcript
+              // re-rasterizes twice a second. Pickers are siblings and every menu
+              // and tooltip in here is a Radix portal, so nothing floating clips.
+              // Tightest descendant is the attachment remove button, which
+              // overhangs its thumbnail by 6px and clears this box's padding by
+              // 4px — keep that slack if the padding below ever shrinks.
+              '[contain:paint]'
             )}
           >
             {imageAttachments.length > 0 ? (

src/renderer/src/components/native-chat/native-chat-composer-containment.test.ts

@@ -0,0 +1,33 @@
+import fs from 'node:fs'
+import { describe, expect, it } from 'vitest'
+
+const composerField = fs.readFileSync(
+  new URL('./NativeChatComposerField.tsx', import.meta.url),
+  'utf8'
+)
+const autocompleteMenus = fs.readFileSync(
+  new URL('./NativeChatAutocompleteMenus.tsx', import.meta.url),
+  'utf8'
+)
+
+describe('native chat composer paint containment (#10481)', () => {
+  it('bounds caret repaints to the composer input shell', () => {
+    expect(composerField).toContain('[contain:paint]')
+  })
+
+  it('keeps the outer composer uncontained so the pickers can overflow it', () => {
+    // The pickers are siblings that render above the shell via `bottom-full`;
+    // containing their parent would clip them.
+    const outerShell = composerField.slice(0, composerField.indexOf('[contain:paint]'))
+    expect(outerShell).toContain('<div className="shrink-0 bg-background">')
+    expect(outerShell).not.toContain('contain:paint')
+  })
+
+  it('lifts both pickers above the contained shell', () => {
+    // The shell is a stacking context now, so it paints at z-index 0 in tree
+    // order — an unlayered picker would lose its drop shadow to it.
+    for (const picker of ['bottom-full left-0 right-0 z-20', 'bottom-full left-3 right-3 z-20']) {
+      expect(autocompleteMenus).toContain(picker)
+    }
+  })
+})

src/renderer/src/lib/pane-manager/pane-cursor-blink-suspension.ts

@@ -12,10 +12,16 @@ import type { Terminal } from '@xterm/xterm'
  * and the pane blinks — redrawing its whole cursor row through
  * `WebglRenderer._updateModel` — until the 5-minute idle timeout.
  *
- * `cursorBlink` is the public option that tears the timer down deterministically
+ * `cursorBlink` is the public option that tears the timer down
  * (`RenderService.handleOptionsChanged` -> `WebglRenderer._updateCursorBlink`), so
  * "a hidden pane does not blink" stops depending on which CSS hid it.
  *
+ * Not unconditional, though: `_updateCursorBlink` resolves
+ * `decPrivateModes.cursorBlink ?? options.cursorBlink`, and DECSCUSR with a
+ * blinking style (`CSI 5 SP q`) pins that DEC mode. On a pane whose shell or agent
+ * has emitted one, parking the option here has no effect and the hidden pane keeps
+ * blinking. Making this deterministic means clearing the DEC mode too.
+ *
  * Resume restores the parked value rather than the settings value, so a pane that
  * was not blinking before the hide never comes back blinking.
  */