I use maximized window gaps, which enforces window gaps on all windows except for exceptions you specify.

• By default it also applies to the lock screen showing black borders around it.
• The splash screen moves around after I login. Here’s a screenshot I got with ksplashqml --test`.

I went to Settings app > Windows Rules > Add New > Detect Window Properties, and clicking the window shows ksplashqml for the class name, but that doesn't work when I try adding it as an exception to maximized window gaps, other window class names exceptions work like slurp.