We've just discovered this bug - not sure if this is the correct place to report it?
If you set TextOptions.TextFormattingMode="Display" on the SyntaxEditor control, or any of its visual parents it causes a severe rendering bug. If you type a line of text into the editor, you'll see that the cursor isn't rendered at the end of the line, but a few pixels back from it. The effect gets worse as the number of characters in the line increases, with the cursor getting further and further back from the end of the line.
Selection is also broken: if you try selecting from the end of the line, the selection will actually start several characters back from the end of the line.