There is a discrepancy in how the two axes are handled inside InspectorOverlay::buildGridOverlay where areas are not always taking the border of the grid container into account. Grid lines are correctly aligned in these situations, but the area borders are not.
Created attachment 425200 [details]
Screenshot of Issue
Created attachment 425201 [details]
Screenshot of Patch v1.0
Created attachment 425202 [details]
Comment on attachment 425202 [details]
r=me, good catch
media/modern-media-controls/macos-fullscreen-media-controls/macos-fullscreen-media-controls-drag-is-prevented-over-button.html has a history of flaky failure on mac-AS-debug-wk2 unrelated to this patch.
Committed r275529: <https://commits.webkit.org/r275529>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 425202 [details].