The fixed backdrop masked the whole viewport; inset it by the sidebar
track parsed off the app frame's inline grid template (the sole inline
gridTemplateColumns in the dsh client, identical on every supported
baseline), so the dialog centers over the main column and the sidebar
stays visible and interactive. An unresolved frame degrades to the
full-viewport mask.
The measure runs in a layout effect (no 0-inset first paint) and, while
open, a MutationObserver on the frame's style attribute follows sidebar
drags, collapse toggles and narrow-viewport re-solves. A per-baseline
compat probe pins the frame seam.
Bowen Liang
b11a1d2151