It looks like our array of column positions is ordered logically (column 0 is the physical right-edge when using RTL), but is always shifted by the physical left border and padding. When computing the breadth of an abs-pos item that covers the padding area (using 'auto'), we need to account for this, since the auto lines are purely in logical coords.
Created attachment 451974 [details] Patch
This patch modifies the imported WPT tests. Please ensure that any changes on the tests (not coming from a WPT import) are exported to WPT. Please see https://trac.webkit.org/wiki/WPTExportProcess
Committed r290032 (247412@main): <https://commits.webkit.org/247412@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 451974 [details].
<rdar://problem/89093512>