Does something exist like "focus-content"?
In GTK (and most other GUI systems) the concept of "focus" is the widget that is going to get keyboard input, the mouse can move around and point to things before a button click, but there is not a similar capability for the keyboard, a widget has "focus" and does not depend on the position of the mouse. So focus is usually on something in the content, like the search entry in the Symbols tab, or the tree. But you need to know it exists. You can see some of the problems around this even for built-in parts of Geany when the contents change in #3678.