Clicking in a panel doesn't focus it but clicking on its "tab" at the top does focus it. This shows when switching from one panel to another, and pressing Cmd+R, for example, to start reading aloud - reading starts in the "old" (previously "focused") panel, even though the new panel is shown in the front. The same applies to switching to Paged View, etc. To reproduce, float two panels and dock them next to one another, then click on the *tab* of the one to focus it, then click into the *body* of the second panel. Now press Cmd+R, etc and you'll (I hope) see that reading aloud starts in the wrong panel.
The visual (very difficult to see, but still "visual") focus border drawn around the focused panel indicates that the focus does not shift when clicking inside the body of a resource but only when clicking on its tab.