Betting the answer will make me slap my forehead and go, “well, duh!”…
Mac Finder windows started showing a side preview pane some months ago when I upgraded the OS (now on 12.4). See label (1) below. I can make the side pane go away by dragging its border right but it’s temporary. Comes right back when I open another window. Can’t seem to find a setting that controls this. I want it to default to no side pane.
Second issue is that previews triggered by the space bar now show in miniature (2), next to a ridiculously huge file name. Not all files: Word files show the file content in the full preview window, most other files (jpeg, png, pdf…) show in the less-useful miniature. Also can’t find a setting that affects this.
You can try to fix the second problem by going to the App “Activity Monitor” and look for “QuickLookUIService(Finder)”.
Mark that, click the Stop-Button (the one with the “X”) and select “Force Quit”.
The process should restart thereafter by itself, latest if you use Preview the next time.
I have noticed this issue keeps reoccurring for me frequently lately, and relaunching the Finder always helps. It probably restarts the same task @Ulli was referring to.
If a problem like that reoccurs frequently, it might be worth to run “First aid” from “Disk Utility” on the HDD/SDD, just to be sure, that everything is all right.