Try this: Go to Tools > Options > Appearance > Colors, and change the setting for "Unknown XML Element" to be Normal text instead of Bold. Do the same for the Block highlighting color. In some proportional fonts, the text moves around when Block highlighting happens (because of the bold text). You are probably clicking right before block highlighting kicks in. Going further with the theory, you might be able to eliminate the problem by turning off Tools > Options > Editing > General > Highlight Matching Blocks.
If this is the problem, I'm not sure there's a fix, other than using a better-behaved Unicode font, such as Consolas.