It seems sideways-lr text frames have a larger than normal overflow area. This additional area has the same size as the overflow area the text frame should have normally. This additional overflow area doesn't cause any observable rendering difference, so it is probably not a big deal, but we probably want to fix it at some point.

This patch adds code to text frame painting to render a translucent red rect covering the overflow area.

This is the testcase file for comparing the visual overflow area.

Screenshot of the testcase page with a patched build.

FWIW, you can see this without needing a special patched build, if you just enable nglayout.debug.paint_flashing and then watch what gets repainted when selecting text in your testcase, for example.