The "selected" panel in TheBrain version 18.104.22.168 seems to be transparent to mouse right-clicks. It should not be.
After having made a selection of thoughts that appear in the "Selected" panel (left side of window), when I can right-click in that panel I will sometimes get the menu for a thought hidden beneath the panel. That is, instead of the menu for the Selected panel appearing—the menu containing "Copy Thought ..." as the first entry—the menu beginning "Open in New Tab" appears. This happens when my mouse is directly over a thought hidden by the panel.
I have noticed this in Outline layout which is my usual working layout.