Under actions and tools, if you Show Left while it’s highlighted, the document pane disappears, if you Select it again, the document pane comes back BUT the Project Menu on the tool bar now has focus. Using Show Right on/off or switching Simple View/Print Layout multiple times doesn’t change the toolbar focus.
This is deliberate behaviour. When you click Show Left and the Document tree opens, focus is given to that tree, as the most likely reason for opening the tree is that you want to take an action of some sort — open a new scene, add a note, etc., all of which require the project toolbar.
When you click Show Left you’ll most likely be presented with a list of currently open notes. In the case the likelihood is not as high that you’ll want to take action in the notes section — write inside a note. It’s more likely that you’ll be looking to read an existing note, so focus does not move to that section.