Hi, Recently, some update or another caused my windows to go horizontal-full-screen when I move a window from one physical screen to another. The best I can tell, it is due to "Screen Edges" turning on "Tile windows". Apparently, if I middle-button click the title bar, it stops that behavior *FOR THAT PARTICULAR WINDOW*. I cannot find a global setting to stop this horrible behavior. I'm sure this change was made years ago because my KDE updates always lag by years, but I sure hope there is a setting for fixing this very inconvenient problem. Hopefully, that setting will be honored when I do my OS refresh in a couple of months, too.

So: how do I prevent horizontal maximization when moving windows from one screen to another?

I think the relevant settings you may be interested in should be in System Settings > Desktop Behavior > Screen Edges.
Note that with those settings on then while you are dragging the window near a screen edge it should only provide a preview of how it will tile the window if dropped at that location, but you should be able to move the window between screens and drop it in the middle of the other screen without it actually tiling the window. That is, only if you drop it when the preview rectangle is visible should it apply the tiling.

If that doesn't cover the problem, then you might examine the effects you have enabled under System Settings > Desktop Behavior > Desktop Effects, and any other scripts you have enabled under System Settings > Window Management > KWin Scripts.

Thank you!

I disabled "Tile windows by dragging them to the left ..." and that seems to have fixed it.
I certainly never went in and turned it on. If I want to maximize width, I just click and
drag the edge. I'm not a fan of automated help from a window manager.