broken selection feedback when selecting bold text in markdown files

open a markdown file with bold markup, e.g.
abc text xyz
try to select the line with the mouse starting at 'abc' and slowly extending to 'xyz'
Observe: if you reach the second '**', the selection of 'text' disappears. In the end you have two disconnected selection ranges.
in addition I see lots of flashing selection changes while dragging the mouse

Merge pull request #124 from Microsoft/weinand-patch-1

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