When editing a large file (1000+ lines) with Line numbers on, the point where the line numbers get an extra digit 999 - 1000) seems to have a bad effect on text selection. The start index of the selection changes spontaneously when using Page Up button.
1) Make sure option 'Display line number margin' is enabled:
* menu 'Settings' -> 'Preferences...', tab 'Editing', check the checkbox 'Display line number margin'.
2) Create a new blank text file.
3) Enter the text 'TEST TEXT' on line 1.
4) Copy this line so you end up with a file of 1100 lines, of which the last one is empty:
(You can also use the example file that I attached)
5) Place the cursor at the start of line 1098 (so column 1 of that line)
6) Use key Page Up while holding SHIFT to increase your selection towards the top. Press it repeatedly, keeping a close eye on the selection start index (the place where the selection begins)
7) Observe: Right around the point where the number of digits in the line numbering margin decreases from 4 to 3 (line number goes from 1000 to 999) the start of the selection is suddenly changed to column 2! The first character of the line is no longer selected!
This behaviour seems to be directly related to the line numbering. If line numbering is off, it disappears.