SUMMARY
If playing a game in full screen stadia, using the chromium browser, pressing the "esc" key results in any panels on the screen showing up -- it should remain hidden behind the full screen chromium.

STEPS TO REPRODUCE
1. Launch any game on Stadia on a screen with at least one panel (e.g. Destiny 2 -- https://stadia.google.com/games/destiny-2)
2. The game will open full screen
3. Press and release the "esc" key

OBSERVED RESULT
The game remains in full screen, and the plasma panel appears in front of it.

EXPECTED RESULT
The game remains in full screen, and the plasma panel stays hidden behind it.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.20.3
(available in About System)
KDE Plasma Version: 5.20.3
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Fedora 33 Packaging w/ Chromium Freeworld was used to reproduce this on nvidia hardware. It has also been reproduced by a friend on KDE Neon packages with AMD hardware, and later on Fedora 33 Packaging w/ Chromium Freeworld and AMD hardware.

This has been brought up a couple of times on reddit:
https://www.reddit.com/r/kde/comments/g7rcas
/pressing_esc_when_playing_stadia_in_google_chrome/
https://www.reddit.com/r/linuxquestions/comments/eaiyvn/fullscreen_mode_chromium_eg_stadia_showing_panel/

I previously mentioned a workaround where you can create an application profile for chrome/chromium to force it to always be on top, which prevents this issue -- but of course has other detriments.