Example:
There is a single, primary screen 1024*768 and another unconnected 1024*768 screen. A window is anchored in the top left corner, its position is accordingly [0,0]. When a new screen is connected and moved to the right side of the existing screen, the desktop gets extended over the second screen so one has a 2048*768 desktop, no issues so far.

But when the second screen is initially connected (or moved after being connected) to the LEFT side of the primary screen, this screen is then no longer at 0,0, but at 1024,0. So windows at position [0,0] suddenly get moved to the other screen while they should be updated along the screen they are currently on… i.e. [1024,0] position so they stay on the initial screen.

This might not be a big issue for the regular desktop setup. But when one has a laptop with the laptop LCD as the primary screen, external screens usually have vastly different resolutions. Maximised windows get thrown onto the other screen an might scale in undesired ways. Or even worse: A beamer is connected and suddenly private notes or other things are thrown onto the new screen and thus visible to everyone watching the beamer.

The desired behavior would be that when changing the screen resolution due to another output connected, kwin checks which screen a window is currently anchored on adds whatever translation this screen gets also to the window positions.

For convenience also an ASCII-Art picture, (needs monospace font)

[ 1              ]
[    WW          ]  (only screen 1 active)
[    WW          ]

Then another screen is connected:
[ 2              ][ 1             ]                               [ 2             ][ 1              ]
[    WW          ][               ]     while it should be:       [               ][    WW          ]
[    WW          ][               ]                               [               ][    WW          ]


Reproducible: Always