sunfish: Fix multi-display region root change

This CL fixes an issue where the panel would not update roots if a
selection was started on a new root by doing the following:
- Fix panel bounds calculations to all be in screen coordinates
- Update the panel bounds whenever a new selection is started, i.e.
the panel is not visible
- Update the panel root and parent before setting its bounds

Demo: https://b.corp.google.com/issues/364718783#comment3

Test: added
Fixed: b:364718783
Change-Id: I9b324aff23744630497f981c2392e3f2b7d45998
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6125434
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Sophie Wen <sophiewen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1403208}