The histogram is measured on settled frames only, so during a drag it
describes the frame from before the gesture while the canvas shows
something newer, and nothing said so. The draft flag stopped at the
render closure.
It now reaches the interface: `canvas-draft` on the window and
`Levels.provisional` for the readouts, both set on every canvas render
from the flag that chose the frame's resolution, and cleared when a
render fails. The histogram panel dims its display reading to half
while a draft is up and brings it back when the frame settles. Dimmed
rather than captioned, because a caption appearing on every drag would
move the column; the raw reading has no frame to lag and is left alone.
dtourolle
4642c77e18