- they need to be separate, both on at once is too much

    --features debug-widget-callstack
      Show callstack for the current widget on hover if all modifier keys
      are pressed down

    --features debug-interactive-widgets
      Show an overlay on all interactive widgets

    Notes:
    - debug-widget-callstack asserts `egui:callstack` feature when enabled
    - Only works in debug builds, compile error w/ release builds

Previous approach was to keep muted content from getting inserted.

Instead, this version alters it's display.  This makes toggling mutes
on and off externally much more stable (the display changes but we
don't have to rebuild content trees)

For now muted content is collapsed to a red "Muted" tombstone w/ a reason.