What a tab’s content is doing, marked in the tab’s leading slot in place
of its icon: loading, or making sound. A sound mark is a control: a
click on it is reported by TabBar::take_mark_click (the host mutes
or unmutes).
A window’s application mark at any size: the app’s own artwork when it
has some, else the app-kind glyph its tab pill wears, scaled up from the
pill’s 14 px design size. Every surface that names a window by its app
(the tab pill, the window switcher, the overview) paints this one mark.