Instructions to reproduce:
Install this update, reboot and try again
No, it's not the crash in sddm.
It's xdg-desktop-portal starting too soon when logging into the desktop, with an incomplete environment. That means that every call to the portal goes to xdg-desktop-portal-gnome instead of -kde. Also any call that causes the portal to start a new process makes it also have an incomplete environment, meaning you get wrong Qt theming, cursor, possibly broken functionality... See the list of duplicate bugs at https://bugs.kde.org/show_bug.cgi?id=460484
On the other hand, the too-early start only happens in the Xorg session, so given that we default to Wayland it's maybe not as urgent.
With plasma-workspace 5.26.2 the xdg-desktop-portal is broken without updating the qt5-qtbase patch collection. It was already updated in rawhide as 5.15.7. Can we include that in this update? Alternatively, backport https://invent.kde.org/qt/qt/qtbase/-/commit/a7bd656cf40b0cf7ee7f96e6c59974804a4894ba to the current qt5-qtbase f37 package
I also experienced konsole and some other windows being reset to their smallest size (testing on f36 via copr). I can also reproduce your issue with widget popups. This is probably a regression after the changes that made popups of panel widgets resizeable by dragging their corners. You should report the bug to KDE directly...
I'm not sure if it should be a blocker.