When you go from having a two-pane layout to a one pane layout, and focus was previously in the pane which was collapsed, focus is lost altogether. Expected behaviour would be that it is moved to the other pane.
Alt+Shift+2 to open two panes.
Open a file in each, and give focus to the right-hand view.
Alt+Shift+1 to go back to one pane.
Now, typing or hotkeys such as Ctrl+P don't work. Workaround is to give the pane focus via clicking or pressing Ctrl+1