What default fonts is qOwnNotes setting as the default for you? Under GNOME it's defaulting to Clear Sans (10) for Default font and monospace (10) for Code Font (under Interface -> Editor -> Editor fonts), which are the defaults we set in the GNOME branding package. Just doing a quick look, I'm not seeing anything in the Plasma branding package to indicate a default font being set, but not sure if @sunnyflunk sets that elsewhere.

Noto Sans (10) and monospace (10) under Budgie
[something illegible/and or missing] (12) and monospace (9) under Plasma

I don't know if it's related, but the same font appears in some windows with Wine and Budgie.

The font is called "Ani". I have no idea why qOwnNotes would be trying to use it, but I'd change it to something more sane, Noto maybe?

Capturing a rectangular region with Spectacle works fine here, no issues. Don't know if or why the font would affect Spectacle, maybe some kind of rendering issue caused by the font makes the display manager go haywire?

@centzon400, I suggest changing the font in qOwnNotes and trying again, see if it's still a problem.

Ach! Right after I tried a few of those automated font recognition sites, I though I'd hit the reddits. I posted literally a minute or two before you. It is definitely Ani. Well spotted!

Changing the editor fonts is not really the issue... it is where the font was coming from. Installing qownnotes was the very first thing I did after installation.

Ani is still in the interface, regardless of the option I choose GTK+ etc etc. The editor/markdown side of things is fine.

Spectacle didn't hang my session this time, but it still behaves "oddly" is all I can say.

So perhaps this warrants having a look at the ootb experience of gtk apps, ensuring it's settings something sane (there are gtk app controls in the system settings). However, if the app isn't taking these settings into account (perhaps hard coded to look in locations), that is something else to address.