Drag and drop allows impossible placement
Using the 1:3.22.0-3 gnome-tetravex package from Debian Buster it is possible to use drag-and-drop to swap adjacent illegally. Everything fine so far
Clicking either tile and dragging it onto the other causes them to swap even without a match
You cannot swap if a swapped tile would result in a mismatch with tiles other than the two tiles being swapped
If there is not a contradiction with tiles other than those being swapped, the invalid placement is still possible