There are two different kinds of fullscreen mode - "tab fullscreen" and
"browser fullscreen".
"tab fullscreen" refers to when a tab enters fullscreen mode via the
JS or Pepper fullscreen API;
"browser fullscreen" refers to the user putting the browser itself
into fullscreen mode from the UI (e.g., pressing F11).
NOTE: Mouse lock is only allowed in "tab fullscreen" mode.
Lock mouse:
left click in either of the two boxes; or
right click in either of the boxes to ensure that it is focused and
then press Enter key. (You could verify that the tooltip window is
dismissed properly by this second approach.)