Code a line, or open a code file. Code is at state A.
Select some text.
Using the mouse, drag the selected text somewhere else.
Select undo or Cmd-Z / Ctrl-Z
 * your text is NOT moved back to where it was before * Code is at State B
Select undo or Cmd-Z / Ctrl-Z
 * Your text is NOW moved back to where it was before * Code is at State A.


It is not clear what State B is or why it happens.

If you press ctrl-z only once and save the file, the code is very likely broken! 

With remote ftp sync upload on save, now my site is broken, too.

Possibly related to 18319