Bug: tooltip position is wrong when highlighted text crosses over to next line

[João Manuel Rodrigues]

Steps to reproduce bug:

1. Access a document, say http://sing.ei.uvigo.es/sds/document/119
2. Resize browser window so that some highlighted expression starts near the end of a line and ends on a different line, just after the begining of the line.
3. Click on that expression to show the tooltip.
   Result: the tootip shows on the midpoint between the start and end, which does not intersect any of the highlighted expression.

See attached screenshot (the displayed tooltip is for the expression highlited in red).