SUMMARY

Currently, it is not possible to move a window to a different activity. You must add it to your desired activity, the remove it from the current one. This is pretty unintuitive and cumbersome. 

It would be really helpful to be able to right-click on a window and see an option to move it to another activity directly.


THE CURRENT PROCESS
1. Right-click on a window titlebar
2. Hover over "Show in Activities" until a menu appears.
3. Select the activity you want the window moved to. The menu should disappear.
4. Right-click the window's titlebar again.
5. Hover over "Show in Activities" until the menu appears again.
6. Select the current activity, and the window should disappear.

THE SUGGESTED PROCESS
1. Right-click on a window titlebar
2. Hover over "Move Window to Activity"
3. Select the activity you want the window moved to. The window should disappear.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
- There was a similar request for KDE4 that was never fulfilled. https://bugs.kde.org/show_bug.cgi?id=272768, indicating that this is a long-lived pain point for KDE users.