A fresh thread has nothing to show, but the HUD showed a slab of frosted
glass above the bar anyway. Vibrancy is the window's whole content view, so
it frosts the full rectangle — fine while the band always filled the window,
wrong the moment there is no transcript to fill it. It stays off until there
is something to back.
The sheet had a 12px floor for the same reason: the breathing room above the
first row was added in CSS, so a zero-row transcript still measured 12. It is
folded into the measured height now and only applies when there are rows.
843c3abcc7