When I drag a fullscreen window with alt+clicking the window between monitors (tested on SMPlayer, html or flash videos, and Dota 2, vlc works though) the screen just duplicate into 2. And the screens are not being updated (freezing). Alt tabbing into the window afterward will fix it but it's really annoying.
please attach the output of qdbus org.kde.KWin /KWin supportInformation
Created attachment 93579 [details] qdbus org.kde.KWin /KWin supportInformation There you go, the strange thing is Multi-Head value is set to "no" even if it knows I've got 2 screens
multihead != multiple outputs. Multihead means to have two independend X11 screens (the mouse can cross by default them, but windows never can) I suspect the problem to be "unredirectFullscreen: true" run "kcmshell5 kwincompositing" and UNtick "suspend compositor for fullscreen windows", then try again (in very doubt, restart "kwin_x11 --replace &")
Indeed it is the unredirect fullscreen fault. now the alt dragging works. Thank you Lubking :)
unredirection won't get you much - if you require more free resources, simply suspend the compositor. This is however still a bug - we got to figure whether it's ours ;-)
Support for unredirecting fullscreen windows got removed in Plasma 5.8