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This idea is summarize of three problems with one solution.

1. Many people asks to allow to have different plasma activity on different screen
2. In past Plasma offer overlay mode (transparent set of plasmoid/activity)
3. Allow to create floating plasma view/plasma activity using plasma UI (not plasmawindowed console with single widget)

About third point. I asks to allow to create floating plasma window/plasma activity using activity panel (SUPER+Tab). Additional won if you allow to create separate activity, so widgets placed in this floating plasma window will have different favorites/connected files/history/etc. Use case for this is one - to allow for example create clock plasmoid and make it's window always above others.

About second point. I think we should offer two builtin overlays: control overlay and statistics overlay. On control overlay we will have plasmoids such like: fast user switcher, logout, notes, history of last opened documents/programs and button to open ksysguard. On statistics overlay we have only preview plasmoids about our hardware statistics.

About first point. Simple: create plasma floating window, disable it's border and maximize it on another screen.