>> I
never know on which monitor an application will launch in after I start it.
Funny, this is one of my biggest complaints about KDE since the 4.x days. I use a thunderbolt dock for my 2 external displays, and occasionally will lose power or accidentally wiggle the laptop too much, power down the displays, or just want to save on power/heat to power off myself, all hell breaks loose. Often once they come back up, it'll do some random combination of one or all of 1) reverse the order of the displays, 2) reset all settings of my displays (like resolution/hz), 3) jumble every damn window of mine to random places, 4) move everything to one display only, and 5) resize all my windows only on my 3rd display only to be 1px wide. This is a 4k monitor with not very thick window borders to begin with to do so!
Freakin' maddening, particularly #5 makes me want 5 finger death punch my display when I have to manually resize a few dozen windows on it! Even worse about #5 above, once I resize the window, if I try to move the window vs. resize it, it snaps like a rubberband to be 1px wide again. What the hell would you even begin with to troubleshoot *that*?. Is it kde base, kwin, video drivers/card, hdmi vs. displayport, mesa, or xorg? They all blame each other, or me for being fancy with my hardware, so I gave up trying.
Trying to describe these to resolve in a bugtracker only nets me somewhere between a confused wtf sort of answer or "ohh, mr fancy with 3x 4k displays, none of us have that to test with, sorry not sorry". It's like that Dave Chappelle episode teaching customer service at the printers, "If the customer says they have windows, tell them you only know mac. If they say they have mac, tell them you only know windows. If they say they have windows and mac, tell them you only know linux!"
I've tried gnome shell (hate it), cinnamon (tolerate it, has it's own display issues), mate (like it, but missing functionality I'm used to in Cinnamon and/or KDE), and unity (hated it, thankfully rip), still come back to KDE even with maddening persistent multi-monitor flaws for well over a decade now. I've tried xfce, i3, some others, but just never "got it". I am not a developer and understand why they prefer the tiling - I just don't work that way 98% of the time. I move windows lots, and keep a lot of apps open buried in layers, calling them forth from my dock as needed, for that KDE + Cairo-Dock work great, so long as I keep the displays powered up 24/7.
-mb