cmux Debugging

Debug event log

Put debug event instrumentation (keys, mouse, focus, splits, tabs) in the unified DEBUG build log. This is not a requirement to log every new code path; most probes belong to a dogfood debug loop and are removed before merge.

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

• Untagged Debug app logs to /tmp/cmux-debug.log; tagged (./scripts/reload.sh --tag <tag>) to /tmp/cmux-debug-<tag>.log.
• reload.sh writes the current log path to /tmp/cmux-last-debug-log-path and the selected dev CLI path to /tmp/cmux-last-cli-path, and points /tmp/cmux-cli and $HOME/.local/bin/cmux-dev at that CLI.
• Implementation: Packages/macOS/CMUXDebugLog/Sources/CMUXDebugLog/DebugEventLog.swift. App shim: Sources/App/DebugLogging.swift. Both are #if DEBUG, so every call site must be wrapped in #if DEBUG / #endif.
• cmuxDebugLog("message") timestamps and appends in real time. A 500-entry ring buffer backs it; CMUXDebugLog.DebugEventLog.shared.dump() writes the full buffer to file.
• Key events are logged in AppDelegate.swift (monitor, performKeyEquivalent); mouse/UI events inline in views (ContentView, BrowserPanelView).
• Stable event prefixes: focus.panel, focus.bonsplit, focus.firstResponder, focus.moveFocus, tab.select, tab.close, tab.dragStart, tab.drop, pane.focus, pane.drop, divider.dragStart.

Profiling

Profile a tagged build by attaching to its pid (xctrace record --attach <pid>, sample <pid>). Never use xctrace --launch or Instruments' launch mode on any cmux bundle, and never quit, kill or relaunch the user's running cmux (com.cmuxterm.app): it holds their live agent sessions, and on 2026-09-26 a suspected profiler relaunch took five of them down.

Debug menu

DEBUG builds get a Debug menu in the macOS menu bar. When the user says "debug menu" or "debug window" they mean this, not defaults write.

Debug > Debug Windows holds panels for tuning layout, colors, and behavior, listed alphabetically with no dividers. To add one: create an NSWindowController subclass with a shared singleton, register it in the "Debug Windows" menu in Sources/cmuxApp.swift, and back it with a SwiftUI view using @AppStorage bindings for live changes.

Runtime pitfalls

• Custom drag-and-drop UTTypes 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() in Sources/TerminalWindowPortal.swift runs on every event including keyboard. Add no work outside the allowsPortalPointerHitTesting guard in performHitTest.
• TabItemView in Sources/ContentView.swift uses Equatable plus .equatable() to skip body re-evaluation during typing. Do not add environment/store/binding reads without updating == and keeping .equatable() at the call site.
• TerminalSurface.forceRefresh() in Packages/macOS/CmuxTerminal/Sources/CmuxTerminal/Surface/TerminalSurface+ForceRefresh.swift runs on every keystroke. No allocations, file I/O, or formatting.
• SurfaceSearchOverlay must be mounted from GhosttySurfaceScrollView in Sources/GhosttyTerminalView.swift, not from SwiftUI panel containers.
• Views below a LazyVStack / LazyHStack / List / ForEach boundary receive immutable snapshots plus closures, never an observable store.
• Functions called from SwiftUI body must not mutate state or schedule store writes.
• Foundation, SwiftUI, AttributeGraph, and WebKit semantics change between macOS majors. Test on the reporter's macOS before declaring a user repro disproven.

Detailed references

• references/debug-event-log.md: when to add probes and how to name them.
• references/runtime-pitfalls.md: read before touching terminal rendering, hit testing, tab rows, list virtualization, search overlay layering, or OS-version-sensitive code.