I have spotted that when doing the following process on Windows:
1) Open Codeblocks
2) Open text file in codeblocks
3) Open the same text file in an external editor (on Windows notepad++ for example)
4) Both editors have to be seen on the screen
5) Modify the text in the external editor and save
6) Left mouse click in codeblocks.
7) The reload dialog appears, say reload yes
The wxScintilla code selects text based on the mouse movements.
The wxScintilla::OnMouseLeftUP(...) in wxScintilla.cpp is NOT called after you release the left mouse button in step 6 and as such the code thinks that you still have the left button pressed and therefore are wanting to select the text in step 8, EXCEPT as the file has been modified the file is reloaded and the text selection should not occur.
Sorry, but I am not familiar enough with the code at this point in time to figure out why or what to do next.