#13 highlighting of a chain can miss a link

[Mark A. Mandel]

Create a chain of two tokens as a single tag and have
that chain highlighted. Put the mouse cursor on one of
the links in the chain and drag it a little, staying
vertically within the highlighted area and horizontally on
the same character. The highlighting will disappear from
one of the links.

The coloring may go odd, too. E.g., the de-highlighted
string may appear in its proper color except for the
letter dragged on, which is black.

This reproduces reliably as described. If I drag across
several characters, reproduction is flakier.

[Mark A. Mandel]