Created attachment 240526 [details] Testcase (needs prefixfree.js) transform-style: flat doesn't flatten the 3d transform and should.
Happens because the flattening element doesn't get composited.
Created attachment 462720 [details] test case 2
It seems to show 'square' in STP162 and WebKit ToT. Although still broken in Safari 16.3 but I think we can mark this as "RESOLVED CONFIGURATION CHANGED". Thanks! Tested using - "test case 2"