The popup is horizontally aligned with the click location, and
vertically close to event (slightly on top of it, from below).
If the click occurs too much on the left or the right side of the
screen, the horizontal position is shifted accordingly, so that it is
never displayed outside of the screen. Note, however, that the small
arrow supposed to point the related event is not shifted as well as of
now (not as straightforward since it is a CSS pseudo-element).
This commit also removes the ability to display the popup _above_ the
event in case the click occurs too close to the bottom of the screen.
This feature should be reintroduced by a later commit once it will have
been fixed :).
3c848fe47c