[norinus]

ELI5

While you are typing a Korean syllable in a terminal, the cursor used to vanish — it came back only when you finished the syllable, or when you clicked away. It was never gone, just hidden underneath the little box that shows the half-typed character. This draws the cursor back, at the end of that box, looking exactly like the one it stands in for.

What Changed

The preedit caret. xterm renders an in-flight composition into .composition-view, an opaque DOM overlay positioned over the cell the renderer draws the cursor in. The cursor is painted on the WebGL/canvas layer, so it cannot show through. terminal-ime-preedit-caret.ts mirrors it in DOM at the end of the preedit, reading the pane's own cursorStyle (block / bar / underline), cursorBlink, and cell metrics so it matches rather than invents a look.

Two implementation notes worth keeping, both recorded as comments:

• the caret is an ordinary in-flow element. A ::after computed correctly but never painted inside this overlay, and an absolutely positioned one falls outside its box and is not painted either
• xterm gives the view direction: rtl with unicode-bidi: isolate, under which the first child is the rightmost. Placement follows the resolved direction rather than assuming append puts the caret last

Anchor scan caching. resolveCursorAgentImeAnchor walks visible rows bottom-up reading up to cols cells per row. A Hangul IME fires compositionupdate per jamo, so that O(rows×cols) walk ran two to three times per typed character. The result is derived from the cursor position and the viewport contents: the cursor is cheap to compare so it is the cache key, and the contents invalidate by event (onWriteParsed / onCursorMove).

Redundant focus. setActivePane computed changed but used it only to gate onActivePaneChange; terminal.focus() ran unconditionally, so React effects re-activating the already-active pane refocused it — 21 calls over 30s of typing. pane-active-focus.ts skips the call when the target textarea already holds focus.

Why

The missing cursor is the user-visible one. The other two are waste that this investigation surfaced, and they belong here because they are the same code path.

Being straight about their size: the anchor scan measured 45–96µs per jamo, and re-focusing an already-focused element is a no-op in a single-pane layout. Neither is something a user feels today. The focus one is a live hazard the moment focus can actually move mid-composition (split panes, chat↔terminal), which is why it is fixed rather than left.

Linked Issue

Fixes #16949

Visual Proof

The caret is a new visible element. Verified in the running app while a syllable composed:

	value
caret size	8×16px — exactly one cell
position	viewX + 14 — immediately right of the preedit
advance per commit	315 → 331 → 347, one syllable each
blink	orca-ime-preedit-caret-blink, 1.2s step

Before, the same probe found no cursor element at all for the whole composition.

Testing

• I manually tested these changes locally
• Automated tests added/updated, or explained why not below

pane-active-focus.test.ts is new and covers the focus decision including the regression it guards. terminal-ime-candidate-anchor.test.ts gained cases for cache invalidation on write and on cursor move, so a stale anchor cannot come back.

The caret's placement and geometry were verified by driving a composition over CDP and reading the element's rect and computed style, then screenshotting it. That is not automated here: it needs a live xterm with a real composition overlay, which the node-environment suite does not provide.

tsc --noEmit (web project)   0 errors
oxlint (repo)                0 errors
vitest src/renderer/src/lib/pane-manager/   71 files, 785 passed, 1 skipped
pnpm run build:desktop       passes

Platform: developed and measured on Windows 11. The overlay behavior is xterm's own, so the missing cursor should reproduce identically on macOS and Linux, but the fix is unverified there by me. pnpm test could not run in my environment — the ensure-native-runtime --runtime=node prestep fails to rebuild node-pty for Node 24 — so vitest was invoked directly.

AI Disclosure

Written with Claude Opus 5 (Claude Code). The geometry figures above were read from the running app over CDP.

Review

Cross-platform — no platform branch. The caret reads xterm options and cell metrics, both platform-neutral. currentColor tracks the overlay's own foreground, which xterm keeps legible against its ground, so it works in both themes.

SSH / remote — renderer-local throughout. No transport, pty, or execution-host code is touched.

Agents / integrations — the Cursor Agent anchor branch is unchanged in behavior, only cached. No provider-specific naming introduced.

Backwards compatibility — PaneManager.setActivePane keeps its signature, its return, and its onActivePaneChange firing condition. The only behavior change is skipping focus() when the target already has it; a terminal without an open textarea, or one whose focus is elsewhere, is focused exactly as before.

Performance — the anchor cache removes the repeated buffer walk. getComputedStyle in the caret path runs once per composition update, not per frame.

Security — no new IPC, no new input path, no parsing of user content.

Known gaps

• After a syllable commits, xterm's real cursor still marks the pre-commit cell until the echo returns (19ms in a plain shell, 29ms in an agent pane by this repo's own probe), so the cursor briefly appears to step back before moving forward. Holding the caret across that gap was implemented and reverted: it could not be verified without risking a caret that outlives an echo that never arrives, and an unverified cursor is worse than a brief one.
• Separately, several rules in terminal.css are scoped under .pane-manager-root, a class that no longer exists in the DOM — including two IME fixes (.xterm-helpers { left: 0 } for candidate-window placement, and caret-color: transparent for a stale Windows caret), which are therefore inert today. The new rule here is scoped to .xterm-container to match the actual DOM. Re-enabling the dead ones is its own change with its own regression risk and is left alone.

[coderabbitai]

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Configuration used: Repository UI

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• src/renderer/src/lib/pane-manager/terminal-ime-preedit-caret.ts

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📝 Walkthrough

Walkthrough

The PR adds pane activation focus helpers that skip redundant terminal focus calls. It adds a DOM caret for xterm IME composition views with block, bar, underline, RTL, theme color, and blink support. It integrates the caret into IME anchor handling. It caches resolved Cursor Agent anchors by cursor and viewport state and invalidates the cache after terminal writes or cursor movement. Tests cover focus decisions and anchor cache behavior.

Merge Risk: ⚪ Minimal · up to 79e32

This localized renderer change restores the cursor during CJK preedit and reduces redundant work without changing transport, permissions, or external interfaces; no actionable merge-blocking risk remains after normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 8 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: restoring cursor visibility during CJK preedit composition.
Description check	✅ Passed	The description is mostly complete. It includes the main required sections, linked issue, visual proof, testing details, AI disclosure, review notes, and known gaps. The Agent skill boundary and check…
Linked Issues check	✅ Passed	The changes satisfy issue #16949. They add a visible preedit caret for CJK composition, cache Cursor Agent anchor scans with write and cursor-move invalidation, avoid redundant terminal refocusing, an…
Out of Scope Changes check	✅ Passed	All changes are related to issue #16949. The caret implementation, anchor-scan caching, focus behavior, CSS, and regression tests directly support the reported cursor-visibility problem and its relate…

Full details: Description check

Explanation

The description is mostly complete. It includes the main required sections, linked issue, visual proof, testing details, AI disclosure, review notes, and known gaps. The Agent skill boundary and checklist sections are not included, but these omissions do not prevent review of the change.

Full details: Linked Issues check

Explanation

The changes satisfy issue #16949. They add a visible preedit caret for CJK composition, cache Cursor Agent anchor scans with write and cursor-move invalidation, avoid redundant terminal refocusing, and add focused regression tests.

Full details: Out of Scope Changes check

Explanation

All changes are related to issue #16949. The caret implementation, anchor-scan caching, focus behavior, CSS, and regression tests directly support the reported cursor-visibility problem and its related improvements.

• Fix all pre-merge checks with AI

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Actionable comments posted: 1

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d7a03ff2-8feb-4acb-b4f5-a3fb0ea9bc76

📥 Commits

Reviewing files that changed from the base of the PR and between 25fa6a9 and f044bb1.

📒 Files selected for processing (7)

• src/renderer/src/assets/terminal.css
• src/renderer/src/lib/pane-manager/pane-active-focus.test.ts
• src/renderer/src/lib/pane-manager/pane-active-focus.ts
• src/renderer/src/lib/pane-manager/pane-manager.ts
• src/renderer/src/lib/pane-manager/terminal-ime-candidate-anchor.test.ts
• src/renderer/src/lib/pane-manager/terminal-ime-candidate-anchor.ts
• src/renderer/src/lib/pane-manager/terminal-ime-preedit-caret.ts

Included review availability: Your plan provides up to 10 included reviews per hour; 8 remain after this review.

[nwparker]

Closing as superseded: issue #16949 is fixed by merged PR #17170 (commit 17170), with acceptance coverage documented in #17172.