This was discovered while testing the fix for JDK-8261840. The fix for that bug exposed a preexisting bug when positioning context menus on Windows with 125% pixel scaling (it doesn't reproduce with any other scaling value that I tried). With the original test case for JDK-8228363, and TOP as the value of side, the initial menu is positioned slightly lower (by a few pixels) than it should be. It is correct the second and subsequent times the context menu is opened.

As indicated in the PR for JDK-8261840, this is not a regression, but is also reproducible with JavaFX 16. See:

https://github.com/openjdk/jfx/pull/410#issuecomment-823005933