The Pager widget for showing virtual desktops expands in size upon plugging in an external monitor. In this example, it was a 1080p monitor plugged into a laptop running 4k, if that matters. KDE Plasma Version: 5.12.0 KDE Frameworks Version: 5.42.0 Qt Version: 5.9.3 Kernel Version 4.13.0-32-generic OS Type: 64-bit
Created attachment 110448 [details] Before monitor connection, normal size.
Created attachment 110449 [details] After monitor connection, larger size.
This is expected and correct behavior. The pager's base aspect ratio is the bounding box of all monitors. Otherwise it can't show their content.
(Note: there's a "Only the current screen" checkbox in the settings, too.)