
it happens if you're in "low performance mode" (which is, you know, the one that turns off a bunch of the stupid graphics effects on the menus that make it run like shit)
quoting myself, from the issue I opened last year about it:
The code for the
PartnerEventOverlayContainer(insteamloopback.host/sp.js?chunkhash=3caaf6adbbbccd5f08bc) has a reference toBLowPerfMode, the return value of which is then passed to the parent of something with aclassNameofTransitionWrapperas the propertiesappear,enterandexit. This tipped me off to test it, and this is the bit that's different; I always run Steam in low bandwidth/low performance mode (because I find the performance in the usual mode with all the images and blur effects too slow, even on beefy hardware), and I did so with my Steam Deck too.Turning off "Low Performance Mode" in Desktop Steam's settings (I'm not sure the setting is exposed in new big picture/the normal Steam Deck UI) suddenly makes the blurred background disappear as it should on my Steam Deck. Likewise, turning it back on reproduces the issue.
I suspect this is enough information for internal Valve folks to find the actual underlying bug (presumably with zero-duration exit transitions?) and patch it up.