Originally reported in issue 807584, here condensed to describe the same problem for <dialog> and Fullscreen.

The original problem existed for <dialog>, and was introduced for Fullscreen with https://chromium.googlesource.com/chromium/src/+/b078ac342807ec59b7b994d8e59145d698337e72. The issue is that if some text is selected, and that part of the tree is made intert by not being inside the modal dialog or not being inside the fullscreen element, then if one interacts with stuff inside the dialog/fullscreen element, the selection in the inert tree isn't necessarily dismissed.

https://software.hixie.ch/utilities/js/live-dom-viewer/saved/5802 is a simple demo demonstrating this for <dialog>. Note how interacting with the *text* inside the open dialog dismissed the selection, but clicking the image does not. Clicking in the on the gray overlay outside of the dialog also dismisses the selection.

This isn't spec'd anywhere, but a sensible behavior would be that inertness due to dialog and Fullscreen have no effect on text selection, that the selection is dismissed in exactly the same situation as if nothing were inert.