Check for selection's actual appearance

[MaxArt2501]

It may happen that a selection may comprise just white space that gets collapsed, thus producing no visible selection. This leads to range.getClientRects() to produce an empty array of rects, so the library has no way to position the popover.

It may happen whn double-clicking outside the area that triggers the popover, or by modifiying the existing selection.

A check has to be performed on the constrained range prior to re/draw the popover.