Created attachment 456177 [details] test case See attached test case. The page appears blank despite having text on it. Selecting the text or resizing the window makes the text appear properly. The issue happens in the shipping Safari as well as the r292110 build on my MacBook Air (M1, 2020) running 20G417.
You may have to reload the test case a few times to get it to work when loaded from bug-238575-attachments.webkit.org -- it should work consistently if you download it, though.
<rdar://problem/91377141>