Commit b279c2d2 was breaking a specific use case, which I oversaw:
when space bar activates the move tool, you may want to release the
space bar while mouse button is pressed, and expect to still be able to
move the layer/selection/guide, but releasing space was stopping the
move immediately. The move tool must only be deactivated when both space
and button 1 are released, and the move itself must continue as long as
button 1 is pressed (when started while space was pressed).

As a nice side effect of this commit, panning and canvas rotation are
also improved since now they can be continued while releasing space
(respectively shift-space) if mouse button 1 was pressed, and up until
the mouse button is released. Pressing space again, then releasing the
mouse, back and forth, also work as expected (i.e. move tool stay
activated though the move stops; and panning or rotation continue).

Of course now we don't get anymore panning/rotation stuck while neither
space nor mouse buttons are pressed (which was the original bug). At
least one of these need to stay pressed for panning/rotation/move to
stay activated. And initial activation is obviously always through
(shift-)space only.