Diagnose a cmux UI that won't switch tabs without touching it

When a user says they can't switch terminals in cmux, check it from the outside before suggesting a restart. /Applications/cmux.app/Contents/Resources/bin/cmux ping returns PONG if the app's socket is alive. cmux tree --all lists every window, workspace, pane and surface, and shows which surface is [selected] versus the caller ("◀ here"), so you can see whether focus moved somewhere unexpected. sample <pid> 3 on the main process shows whether the main thread is blocked or idle: mostly kevent/poll means not hung, so look at input or focus, not a deadlock. macOS has no timeout; wrap CLI calls with perl -e 'alarm shift; exec @ARGV' 8 cmd. cmux focus-panel --panel surface:<id> and select-workspace switch focus from the CLI as a workaround. Dozens of open browser and file-preview surfaces in one pane can keep CPU at 40-60% from SwiftUI updates.