Panel: Revert making the mask smaller by 1px
requested to merge ngraham/plasma-workspace:backport-44aa2e1d87be13e8d624ae8c637d074d39a2e9da into Plasma/5.26
This fixes 460896, but breaks the fix for 417511. It will be re-fixed again by https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/644, without any negative side effects.
BUG: 460896 CCBUG: 417511 FIXED-IN: 5.26.4 (cherry picked from commit 1abe1147)