Doing something like document.querySelector("h1").getBoundingClientRect() or document.querySelector("h1").getClientRects() when "h1" is an element inside a flow thread will return invalid rectangles.
Created attachment 181091 [details]
Although I can not reproduce the bug, I find another one. This is the test case what I used for trying to reproduce. I'm going to open a separate bug for the new bug, closing this one.