Measured at 424x439: the three header actions were 25x21px. That
matters more than it looks, because with the panel spanning the whole
width the scrim underneath it has no uncovered pixels at all -- so the
close button is the only pointer route out of a full-screen surface,
and it was below the 24x24 floor in one dimension.
Sized only in overlay mode. Inline these sit in a 320px column beside
the content, where a mouse is what reaches them and 44px of header is
44px the queue does not get.