Opening a popover from inside another popover (e.g. the branch picker's
new filter menu, when the picker is hosted in the git panel or title
bar) corrupted the frame-caching bookkeeping for whichever cached view
contained them. The corruption was invisible until a later, unrelated
redraw reused that view's cached subtree, at which point the window
crashed (debug) or could misroute input events (release). Fixed by
processing deferred draws in place so their recorded positions remain
valid across frames; added a regression test reproducing the crash via a
cached panel hosting nested popovers.

Release Notes:

- Fixed the branch picker menu in the Git Panel, which was previously
broken due to popovers-within-popovers not being possible in GPUI.