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By write app history I mean not history of opened files, but action.

Firstly, it's possible to gather information about what user do with app. Application could send these information by themselves. Plasma should use similar technique like web browser. App can only register/send info about action, when user interact with it. On X11 it's possible. We could check what pid is related to which window and currently window managers probably grabs each events.

For what? Imagine user go to entertainment activity, click on Steam icon in tray and click friends button. Now Steam inform KActivityd user click on Friend button. When user often uses this button, user may right click on entertainment activity in activity switcher and select Steam: Friends. I known, that this example isn't very good, because Steam icon is in tray, but imagine user will select an person in friends dialog very often, so when he relaunch Steam, he could right click on entertainment activity in activity switcher and select he/she friend to open chat window.

Of course, app could always register action such like watch a promotional movie/banner (advertisement) or something, but app vendors probably tend to make apps as most usable as can and app could send info about event only in certain cases.