Glue terminal frames to live window resize ticks - #11323
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…s in-tick During a live window resize, the portal's didResize handling schedules its geometry pass through the main queue, so every tick commits with the previous tick's hosted frames and the terminal trails the window edge for the whole drag. Ghostty hosts surfaces directly in the view hierarchy and has no such gap. Test-only commit so CI shows red, the fix follows. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
During a live window resize the portal observed NSWindow.didResize on the main OperationQueue and then coalesced its geometry pass through DispatchQueue.main.async, so hosted terminal frames were written at least one runloop turn after each resize tick committed. Every tick painted the window's new size with the previous tick's terminal frames, and the terminal visibly trailed the window edge for the whole drag. Ghostty hosts surfaces directly in the view hierarchy, so its frames land in the same layout pass; cmux's portal indirection is why cmux felt slower. Observe didResize with queue nil (delivered synchronously inside setFrame, while the tick's transaction is still open) and, while a live resize is active, run the full geometry pass synchronously instead of scheduling it. The pass forces subtree layout first, so it reads final anchor frames (bonsplit re-imposes divider fractions in the same layout pass), writes hosted frames in the same commit as the window's new size, and pushes grid-changing surface resizes in-tick. All synchronous-redraw paths are already gated off during live resize, so the pass cannot reach the open-transaction Metal displayIfNeeded wedge; re-entrant echoes terminate in currentlySynchronizingPortalId and the geometry signature check. Non-live-resize ticks keep the scheduled, coalesced path. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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📝 WalkthroughWalkthroughWindow resize notifications now deliver synchronously on the main thread. Live resize synchronizes hosted portal frames within the resize tick, while reveal redraws defer to the next main-queue turn. A regression test verifies frame synchronization timing. ChangesLive Resize Portal Synchronization
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟡 Moderate · up to During live window resizing, revealing a terminal can still trigger a display refresh before the resize transaction commits, which may wedge rendering or leave the revealed terminal stale or blank. The PR is not merge-ready until the refresh is tied to a transaction-safe completion signal or otherwise proven safe. Sequence Diagram(s)sequenceDiagram
participant NSWindow
participant ResizeObserver
participant TerminalWindowPortal
participant HostedTerminalView
participant MainQueue
NSWindow->>ResizeObserver: emits resize notification
ResizeObserver->>TerminalWindowPortal: delivers notification on main thread
TerminalWindowPortal->>HostedTerminalView: synchronizes frame within resize tick
TerminalWindowPortal->>MainQueue: defers reveal redraw during live resize
Suggested reviewers: Important Pre-merge checks failedPlease resolve all errors before merging. Addressing warnings is optional. ❌ Failed checks (2 errors, 2 warnings)
✅ Passed checks (11 passed)
Full details: Description checkExplanation The description provides a detailed and relevant summary, including the cause, fix, safety considerations, and regression test. It does not follow the required template because it omits the required Testing, Demo Video, Review Trigger, and Checklist sections. Full details: Cmux Swift Actor IsolationExplanation PASS. The production diff changes only an existing Full details: Cmux Swift Blocking RuntimeExplanation The production diff materially expands delayed dispatch. Before the PR, the reveal path refreshed immediately when Resolution Replace the live-resize reveal's one-turn Full details: Cmux Browser Automation Off-MainExplanation PASS: The PR changes only Full details: Cmux Expensive Synchronous LoadExplanation PASS: The PR changes only terminal portal geometry and a regression test. The new main-thread live-resize path calls Full details: Cmux Cache Substitution CorrectnessExplanation PASS — The PR changes live-resize geometry observation and deferred surface redraws in Full details: Cmux No Hacky SleepsExplanation PASS: The PR changes only Full details: Cmux Algorithmic ComplexityExplanation The PR routes every live-resize Resolution Keep the synchronous notification required for frame correctness, but implement the live-resize pass as one linear plan. Force hierarchy layout once per tick, take one snapshot of the entries and computed target frames, and update hosted views from that snapshot. Do not call Full details: Cmux Swift ConcurrencyExplanation PASS — The PR does not introduce a prohibited legacy async pattern. The production diff changes an AppKit Full details: Cmux Swift `@Concurrent`Explanation The PR changes only Full details: Cmux Swift Package BoundariesExplanation PASS. The production diff only changes scheduling and redraw behavior in the existing
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…tick The synchronous in-tick pass runs inside the resize tick's still-open transaction, where the reveal branch's refreshSurfaceNow reaches ghostty's Metal drawFrame via displayIfNeeded and wedges on a present only that transaction can commit. A reveal cannot skip its redraw outright (the surface would sit blank until later churn), so route it through the existing one-turn deferred refresh instead. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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1 issue found and verified against the latest diff
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<file name="cmuxTests/TerminalWindowPortalLifecycleHiddenRefreshTests.swift">
<violation number="1" location="cmuxTests/TerminalWindowPortalLifecycleHiddenRefreshTests.swift:169">
P3: This new non-UI regression test in cmuxTests is added to an XCTestCase suite, but the repo's test policy prefers Swift Testing (@Suite/@Test) for new/touched non-UI tests, even inside files that already host XCTest suites. Move this test to a Swift Testing suite (the shared fixtures would need to be reachable from it) rather than growing the XCTest extension.</violation>
</file>
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| /// the whole drag (Ghostty hosts surfaces directly in the hierarchy and | ||
| /// has no such gap). | ||
| @MainActor | ||
| func testLiveResizeTickSynchronizesHostedFrameWithinTheSameTick() throws { |
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P3: This new non-UI regression test in cmuxTests is added to an XCTestCase suite, but the repo's test policy prefers Swift Testing (@Suite/@test) for new/touched non-UI tests, even inside files that already host XCTest suites. Move this test to a Swift Testing suite (the shared fixtures would need to be reachable from it) rather than growing the XCTest extension.
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Check if this issue is valid — if so, understand the root cause and fix it. At cmuxTests/TerminalWindowPortalLifecycleHiddenRefreshTests.swift, line 169:
<comment>This new non-UI regression test in cmuxTests is added to an XCTestCase suite, but the repo's test policy prefers Swift Testing (@Suite/@Test) for new/touched non-UI tests, even inside files that already host XCTest suites. Move this test to a Swift Testing suite (the shared fixtures would need to be reachable from it) rather than growing the XCTest extension.</comment>
<file context>
@@ -158,6 +158,65 @@ extension TerminalWindowPortalLifecycleTests {
+ /// the whole drag (Ghostty hosts surfaces directly in the hierarchy and
+ /// has no such gap).
+ @MainActor
+ func testLiveResizeTickSynchronizesHostedFrameWithinTheSameTick() throws {
+ let window = makeTestWindow(
+ contentRect: NSRect(x: 0, y: 0, width: 520, height: 340),
</file context>
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Kept as XCTest deliberately: the test depends on the tracked-window/portal/surface fixtures and leak-checked teardown that live on the XCTestCase base class (TerminalWindowPortalLifecycleTests), and every sibling regression in this suite is an extension of it. Porting those shared fixtures to Swift Testing is a suite-wide migration, not something to fork inside this fix PR.
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Inline comments:
In `@Sources/TerminalWindowPortal.swift`:
- Line 2240: Update the reveal refresh path guarded by syncLayout and
isWindowLiveResizeActive so deferSurfaceRefresh/refreshSurfaceNow runs only
after the didResize Core Animation transaction completes. Replace the main-queue
async scheduling with an explicit post-transaction or renderer-safe completion
signal, preserving the existing refresh behavior and guard conditions.
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Sources/TerminalWindowPortal.swiftcmuxTests/TerminalWindowPortalLifecycleHiddenRefreshTests.swift
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| // transaction can commit. Unlike the frame-change branch above, | ||
| // a reveal cannot skip its redraw outright — the surface would | ||
| // sit blank until later churn — so defer it one main-queue turn. | ||
| if syncLayout, !isWindowLiveResizeActive { |
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🩺 Stability & Availability | 🟠 Major | 🏗️ Heavy lift
🔎 Supported by static analysis
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#!/bin/bash
set -e
printf '%s\n' '--- changed hunks ---'
git diff --unified=24 -- Sources/TerminalWindowPortal.swift | sed -n '1,260p'
printf '%s\n' '--- helper and reveal path ---'
sed -n '2160,2270p' Sources/TerminalWindowPortal.swift
printf '%s\n' '--- resize observer path ---'
sed -n '740,850p' Sources/TerminalWindowPortal.swift
printf '%s\n' '--- helper definitions and call sites ---'
rg -n -C 8 'deferSurfaceRefresh|refreshSurfaceNow|isWindowLiveResizeActive|didResizeNotification' Sources/TerminalWindowPortal.swiftRepository: manaflow-ai/cmux
Length of output: 22533
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#!/bin/bash
set -e
printf '%s\n' '--- refreshSurfaceNow definitions ---'
rg -n -C 12 'func refreshSurfaceNow|refreshSurfaceNow\(' Sources --glob '*.swift'
printf '%s\n' '--- displayIfNeeded and renderer boundary ---'
rg -n -C 12 'displayIfNeeded|drawFrame|present' Sources --glob '*.swift' | sed -n '1,260p'Repository: manaflow-ai/cmux
Length of output: 37339
🌐 Web query:
Apple Core Animation implicit transaction commits end of run loop DispatchQueue.main.async ordering before transaction commit
💡 Result:
Core Animation implicit transactions are automatically committed by the system at the end of a run loop iteration [1][2][3]. This occurs because Core Animation registers a run loop observer for the kCFRunLoopBeforeWaiting activity [4][3][5]. When you modify the layer tree (e.g., updating a UIView property) on the main thread, Core Animation detects the change and implicitly creates a transaction if one does not already exist [4][6][7]. Because the Core Animation observer is triggered during the kCFRunLoopBeforeWaiting phase—which happens after standard source processing (including the execution of blocks on the main dispatch queue)—the implicit transaction typically commits after tasks submitted via DispatchQueue.main.async have finished executing in the current run loop pass [4][3][5]. In summary, the ordering is as follows: 1. Tasks queued on DispatchQueue.main.async are executed by the run loop [5]. 2. If those tasks modify the layer tree, they participate in the implicit transaction [7]. 3. Once the run loop has processed sources and blocks, it reaches the kCFRunLoopBeforeWaiting state [4][3]. 4. The Core Animation observer triggers, committing the implicit transaction and rendering the changes [4][3][5]. This mechanism ensures that multiple property changes made within the same run loop iteration are batched together into a single atomic update [1][2][7].
Citations:
- 1: https://developer.apple.com/documentation/quartzcore/catransaction
- 2: https://leopard-adc.pepas.com/documentation/Cocoa/Conceptual/CoreAnimation_guide/Articles/Transactions.html
- 3: https://vbat.dev/behind-the-scenes-of-ui-part-1-uikit
- 4: https://fabernovel.github.io/2021-01-04/uikit-rendering-part-3
- 5: https://www.exchangetuts.com/index.php/order-of-operations-in-runloop-on-ios-1640616784407097
- 6: https://fabernovel.github.io/2021-01-04/uikit-rendering-part-2
- 7: https://nonstrict.eu/wwdcindex/wwdc2011/121/
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#!/bin/bash
set -e
rg -n -C 10 'func forceRefresh|forceRefresh\(|drawFrame|present\(' . \
--glob '*.swift' --glob '*.m' --glob '*.mm' --glob '*.h' --glob '*.zig' \
--glob '*.c' --glob '*.cc' --glob '*.cpp' | sed -n '1,260p'Repository: manaflow-ai/cmux
Length of output: 25612
Use a transaction-complete signal before the reveal refresh.
deferSurfaceRefresh calls refreshSurfaceNow, which invokes displayIfNeeded on the hosted view and surface. DispatchQueue.main.async can run before Core Animation commits the didResize transaction, so the reveal refresh can still execute during that transaction.
Use an explicit post-transaction or renderer-safe completion signal.
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In `@Sources/TerminalWindowPortal.swift` at line 2240, Update the reveal refresh
path guarded by syncLayout and isWindowLiveResizeActive so
deferSurfaceRefresh/refreshSurfaceNow runs only after the didResize Core
Animation transaction completes. Replace the main-queue async scheduling with an
explicit post-transaction or renderer-safe completion signal, preserving the
existing refresh behavior and guard conditions.
Source: Coding guidelines
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The one-turn main-queue hop is the mechanism this file already uses for exactly this hazard: deferSurfaceRefresh was introduced (see the comment on pendingDeferredSurfaceRefreshes) so refreshes requested inside someone else's layout/update pass drain on the next main-queue turn, and the divider-drag paths have shipped on it. The main dispatch queue is serviced by the run loop before the BeforeWaiting observer that commits the implicit transaction only in the next iteration; a block enqueued while the didResize tick is on the stack runs after that tick's commit. This change routes the (rare) mid-live-resize reveal through that same existing mechanism rather than inventing a second transaction-complete signal beside it. If we ever see the wedge fire from a deferred refresh, the right fix is to replace the shared mechanism, not just this call site.
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🤖 web_search executed:
Apple documentation NSWindow didResizeNotification Core Animation transaction completion DispatchQueue.main.async guarantee transaction commit
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@azooz2003-bit, I agree that this call site should reuse the shared mechanism.
However, the documented contracts do not guarantee the required ordering. CATransaction states that an implicit transaction commits when the run loop next iterates. DispatchQueue.main.async only schedules asynchronous main-queue work. It does not guarantee that the work runs after the current AppKit/Core Animation transaction commits.
Therefore, deferSurfaceRefresh can remain the single shared mechanism, but it needs a transaction-safe completion boundary before it calls refreshSurfaceNow. That change protects both the live-resize reveal path and the existing layout-update callers.
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During a live window resize, cmux terminals visibly trail the window edge; Ghostty stays glued. The cause is timing, not rendering: the portal that hosts the real terminal views observed
NSWindow.didResizeon the main OperationQueue and then coalesced its geometry pass throughDispatchQueue.main.async, so hosted frames were written at least one runloop turn after each resize tick committed. Every tick painted the window's new size with the previous tick's terminal frames. Ghostty hosts surfaces directly in the view hierarchy, so its frames land in the same layout pass; the portal indirection is why cmux felt slower.Fix: observe
didResizewithqueue: nil, which AppKit delivers synchronously from insidesetFramewhile the tick's transaction is still open, and run the full portal geometry pass synchronously when a live resize is active. The pass forces subtree layout first, so it reads final anchor frames (bonsplit re-imposes divider fractions inside the same layout pass), writes hosted frames into the same commit as the window's new size, and applies grid-changing surface resizes in-tick. Non-live-resize resizes keep the scheduled, coalesced path.Safety: every synchronous-redraw path is already gated off while live resize is active (
synchronizeHostedView's refresh branch andreconcileVisibleHostedViewsAfterGeometrySyncboth skip mid-resize), so the synchronous pass cannot reach the open-transaction MetaldisplayIfNeededwedge documented in the portal. Re-entrant echoes terminate incurrentlySynchronizingPortalIdand the external-geometry signature check. Per-tick work is unchanged: the same pass ran before, one turn late.Commit 1 adds the regression test only (a live-resize tick must glue hosted frames with no queue drain), commit 2 adds the fix, so the test's red-then-green is visible in the commit history.
Dictionary: portal = the AppKit overlay that hosts real terminal views above the SwiftUI hierarchy and copies placeholder geometry onto them; live resize = the window-edge drag AppKit tracks between willStart/didEndLiveResize; tick = one mouse-driven
setFrameduring that drag; anchor = the empty SwiftUI placeholder view whose frame the portal mirrors; transaction = the Core Animation commit that atomically presents one frame's view changes.🤖 Generated with Claude Code
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@codesmith-botwith what you need. Autofix is disabled.Summary by cubic
Fixes terminal frames trailing the window edge during live resize so the terminal stays glued to the resize tick, matching Ghostty's behavior.
didResizewithqueue: nilso the geometry pass runs synchronously inside the tick's transaction; non-live-resize resizes keep the scheduled, coalesced path.displayIfNeededwedge; a reveal can't skip the redraw outright or the surface stays blank.Migration
Written for commit 98b35d3. Summary will update on new commits.
Summary by CodeRabbit
Bug Fixes
Tests