IIRC the bug as originally filed was for non-fission. The STR aren't totally clear, but you have to scroll the iframe to activate it first, and then scroll with the mouse just to the left of the blue box. With that I can reproduce the bug on older macOS bugs, but as of bug 1534549 it seems fixed (for both WR and non-WR). I also tried running in the WR+fission configuration but AFAICT the pid of the iframe is the same as the pid of tab, which means the iframe is not actually getting fission-ized. So I don't think the test is exercising anything different with fission enabled. I would call this fixed by bug 1534549.
Bug 1625173 Comment 5 Edit History
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IIRC the bug as originally filed was for non-fission. The STR aren't totally clear, but you have to scroll the iframe to activate it first, and then scroll with the mouse just to the left of the blue box. With that I can reproduce the bug on older macOS nightlies, but as of bug 1534549 it seems fixed (for both WR and non-WR). I also tried running in the WR+fission configuration but AFAICT the pid of the iframe is the same as the pid of tab, which means the iframe is not actually getting fission-ized. So I don't think the test is exercising anything different with fission enabled. I would call this fixed by bug 1534549.