Specially in the case of comboboxes, those menus could enable scrolling
even if the contents could fit in the work area, and could show blank
space in order to line up the selected item with the combobox.

When such thing happens, take into account scroll_offset when relocating
the menu contents so contents don't jump directly onscreen, and apply
it so scrolling is allowed in the direction that brings the menu onscreen
and blocked in the opposite direction.

Also, wait for cancelling the scroll operation until the touch is released
even if the scrolling arrows disappeared, so the menu item underneath isn't
selected right away.

https://bugzilla.gnome.org/show_bug.cgi?id=678113