It seems that the dialog prompting users for selecting a desktop or a window to share may block other dialog processes such that if the user has multiple prompts at once there can be as little as 1 dialog that will accept user input.

In practice it appears to a user that the portal is frozen and the dialog cannot be closed (as the actual active window may be beneath the frozen window) or moved to reveal the live window.

This can be replicated by starting an application that quickly makes multiple portal screen and window requests at once. I happened to be using obs with a couple screen and window captures configured. When you restart the application all configured captures will access the portal opening (5 in my case) portal requests. Most of the time the top request was unresponsive and could not be moved to reveal the responsive prompt (alt tabbing through them all eventually one responded). Though I compiled some PRs to to use this natively I suspect the flatpak version of obs which includes portal support should be sufficient to replicate this without compiling a lot of code.

Tested on Fedora 33, xdg-desktop-portal-gtk-1.8.0-1.fc33.src.rpm, default gnome wayland session.