When activating the mirror tool (horizontal or vertical) the split bar doesn't appear over the canvas (the option "hide mirror line" is not checked) and you can't move it. The brush stroke are mirroring when drawing. Reproducible: Always
Duplicate of https://bugs.kde.org/show_bug.cgi?id=367629
Thanks for noting that! :) *** This bug has been marked as a duplicate of bug 367629 ***