Using Affinity Designer v1.7 for Windows.
I Found this out because I dragged the navigation panel by accident and wanted to undo the dragging.
I Started out with the navigation panel docked, then I started dragging the panel. I Think this only works if the blue docking hover box is visible. Keep the panel in drag mode by keeping the mouse pressed and then press esc. The panel now docks but you can no longer click anything in the application.
It might also work for other panels but I only tested it with the navigation panel.