Thank you for your explication. OK, the issue does not belong to the Geany project but to the plugin Preview.
Do you know if it is possible to solve the issue in the plugin Preview like this, that after a release of a click on the tab-title of the Preview tab the content of the tab is in focus and not the tab itself?
Plugins should connect to the relevant signal eg "switch-page" or "focus-tab" or "clicked" on the tab or whatever and then implement the behaviour they require.
I have no idea how to program software. Does something exist like "focus-content"?
Thank you for an answer.