I don't see a sense in this behavior. Why the tab-titles of the sidebar have not the same behavior like the tab-titles of the editor?
As I said, the sidebar has __default__ GTK behaviour, the editor tabs have __non-standard__ behaviour that had to be "implemented" in Geany when the editor is focussed deliberately when a document is shown (except when it doesn't, see the infamous #3531) which moves the selection from the tab title to the editor (but I can see the dotted outline on the tab title flash). So the behaviour of the editor notebook is non-standard and an accidental side effect of other code.
If the content-area of the tab-title Documents is in focus: Click 2x on the tab-title Symbols, then click on the tab-title Documents.
I said click once, clicking twice is back to the "select the tab title" behaviour. Once the tab title is selected and the tab changed the tab title of the selection stays on the tab title instead of the content.
You can confirm this is GTK default behaviour with the message window notebook tabs which also do the default thing.
As GTK default behaviour its a bit messy, but default knows nothing about any application specifics and for a11y reasons the default behaviour has to allow for navigation by keyboard arrow keys if there is no mouse, so the fact that the tab title is selected shows that the arrow navigation is going to move between tabs not move within tab contents if its not selected. If its moving between tabs it stays in that mode to allow multiple tabs to be navigated. But it may make it a bit confusing if navigation is mixed mouse and arrows.