`W.Popover` - better display for flex containers

When dealing with flex containers (flex-row + items-stretch) we're getting some problems with how popover ends up locking the trigger element out of the base layout. We should study some way to make `W.Popover` to be as flexible as possible for different use cases, since the ideal scenario would be for it to be layout-agnostic (in the future this should be solved by html's popover element).