cmux-debugging

name: cmux-debugging description: "Debug logging, Debug menu, runtime pitfalls, typing-latency-sensitive paths, SwiftUI list snapshot boundaries, OS-version repros, and local visual iteration for cmux. Use when adding debug probes, diagnosing UI/runtime issues, touching terminal rendering, tab/sidebar list views, drag/drop UTTypes, or using the Debug menu."

cmux Debugging

Debug event log

When adding debug event instrumentation, put events (keys, mouse, focus, splits, tabs) in the unified DEBUG build log. This is not a blanket requirement to add logs to every new code path. Most temporary probes should be added only during the dogfood debug loop and removed before merge.

tail -f "$(cat /tmp/cmux-last-debug-log-path 2>/dev/null || echo /tmp/cmux-debug.log)"

• Untagged Debug app: /tmp/cmux-debug.log
• Tagged Debug app (./scripts/reload.sh --tag <tag>): /tmp/cmux-debug-<tag>.log
• reload.sh writes the current path to /tmp/cmux-last-debug-log-path
• reload.sh writes the selected dev CLI path to /tmp/cmux-last-cli-path
• reload.sh updates /tmp/cmux-cli and $HOME/.local/bin/cmux-dev to that CLI
• Implementation: Packages/CMUXDebugLog/Sources/CMUXDebugLog/DebugEventLog.swift
• App shim: Sources/App/DebugLogging.swift
• Free function cmuxDebugLog("message") logs with timestamp and appends to file in real time from cmux code
• The package implementation and app shim are #if DEBUG; all call sites must be wrapped in #if DEBUG / #endif
• 500-entry ring buffer; CMUXDebugLog.DebugEventLog.shared.dump() writes full buffer to file
• Key events logged in AppDelegate.swift (monitor, performKeyEquivalent)
• Mouse/UI events logged inline in views (ContentView, BrowserPanelView, etc.)
• Focus events: focus.panel, focus.bonsplit, focus.firstResponder, focus.moveFocus
• Bonsplit events: tab.select, tab.close, tab.dragStart, tab.drop, pane.focus, pane.drop, divider.dragStart

Debug menu

The app has a Debug menu in the macOS menu bar only in DEBUG builds. Use it for visual iteration.

• Debug > Debug Windows contains panels for tuning layout, colors, and behavior. Entries are alphabetical with no dividers.
• To add a debug toggle or visual option: create an NSWindowController subclass with a shared singleton, add it to the "Debug Windows" menu in Sources/cmuxApp.swift, and add a SwiftUI view with @AppStorage bindings for live changes.
• When the user says "debug menu" or "debug window", they mean this menu, not defaults write.

Runtime pitfalls

• Custom UTTypes for drag-and-drop must be declared in Resources/Info.plist under UTExportedTypeDeclarations.
• Do not add an app-level display link or manual ghostty_surface_draw loop; rely on Ghostty wakeups/renderer to avoid typing lag.
• WindowTerminalHostView.hitTest() is typing-latency-sensitive. All divider/sidebar/drag routing is gated to pointer events only. Do not add work outside the isPointerEvent guard.
• TabItemView uses Equatable conformance plus .equatable() to skip body re-evaluation during typing. Do not add environment/store/binding reads without updating equality and the call site.
• TerminalSurface.forceRefresh() is called on every keystroke. Do not add allocations, file I/O, or formatting there.
• SurfaceSearchOverlay must be mounted from GhosttySurfaceScrollView in Sources/GhosttyTerminalView.swift, not from SwiftUI panel containers.
• List subtrees with LazyVStack, LazyHStack, List, or ForEach must pass immutable row snapshots plus closures below the boundary. Do not pass observable stores into row views.
• Functions called from SwiftUI body must not mutate state or schedule store writes.
• Foundation, SwiftUI, AttributeGraph, and WebKit semantics can change between macOS major versions. Test on the reporter's macOS before declaring a user repro disproven.

Detailed references

• Read references/debug-event-log.md when adding or interpreting debug log probes.
• Read references/runtime-pitfalls.md before touching terminal rendering, hit testing, tab rows, list virtualization, search overlay layering, or OS-version-sensitive code.