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If you change your screen resolution, plasma does not properly rescale the plasmoids. If you go to a smaller resolution, you end up with plasmoids off the side of the screen. If you go to a larger resolution, you end up with large blank spaces.

Further, if you use two monitors with different resolutions and try to switch activities between the two monitors, especially using the activity bar plasmoid. The activities end up the correct size for the monitors, but the plasmoids in that activity do not change size so you can end up with big empty spaces devoid of plasmoids and/or plasmoids extending off the edge of the screen.

I think the way to solve these problems would be that if an activity's resolution changes, it scales the plasmoids so they stay the same size relative to the activity, rather than the same number of pixels. So a plasmoid that is 25% the height of the activity and is positioned 50% of the way down the activity will remain 25% the height of the activity and 50% of the way down no matter how big or small the activity becomes.