After initiating a MouseDrag(), holding the Alt key while letting go of the mouse button can cause DS to miss MouseDrop(), causing DS to continue to think that you are dragging the mouse. This makes mouse interaction with the entire window completely impossible and any mouse functions will be routed to the atom that you were originally dragging. This bug can be negated by performing a second MouseDrag() and not missing the MouseDrop()
Testing Code:
client
MouseDown(atom/a)
world << "MouseDown([a])"
MouseUp(atom/a)
world << "MouseUp([a])"
MouseDrag(atom/a)
world << "MouseDrag([a])"
MouseDrop(atom/a)
world << "MouseDrop([a])"
Click(atom/a)
world << "Click([a])"
Numbered Steps to Reproduce the Problem:
1) Left-click and hold your mouse down on an atom.
2) Drag your mouse off of the atom.
3) Hold the Alt key while letting go of the left-click button.
Assuming that you were able to reproduce this, despite not holding any mouse buttons, "MouseDrag([a])" will still continue to output.
This seems to happen for me through just Drag>Hold Alt>Release