If there are several lines of code in the Editor, the following steps will cause the rows to be incorrectly highlighted:
1. Enter a script in the editor with a few rows
2. Click 3 times on the first row to highlight the entire row
3. Immediately after the 3rd click, click to the area after the last character of the row beneath
4. Both lines will be highlighted.
(You have to do step 3 immediately after step 2, as part of the same clicking process, or this issue doesn't occur)