[...] the default behaviour has to allow for navigation by keyboard arrow keys if there is no mouse [...]
The keyboard navigation of the tabs in the editor is possible with Ctlr+Page-up/down and nobody is complaining or asking the default GTK behavior. And also in the sidebar one can tab-browse with Ctlr+Page-up/down, so without the mouse, without any problems, no matter if the tab-title or the content-area is in focus.
[...] the sidebar has default GTK behaviour, the editor tabs have non-standard behaviour [...]
Because tab-browsing is possible in the sidebar by keyboard (Ctlr+Page-up/down), so without mouse, what about the idea to change sidebar tabs to a non-standard behavior like the editor?