I assume that is a bug in MacOS rather than in Xojo although I do not remember it happening in older versions. If I do a search on some string (In the example below .FillColor =) I get the locations of all the matches. If I click on one of the matches, and then scroll on the code window a little bit and then click on the same match intending to go back to the matched line again, I get a curious little ghost over the code area. Note the second line 16 which is sitting over where line 22 should be. Click ANYWHERE and the situation corrects itself.
What amused me enough to write this note is that the ghost does not appear when you do a screenshot. The screenshot shows no anomaly. That is why I had to use my iPhone to take the image below.