On Wayland, DragDetect was fired with the post-threshold motion
coordinates instead of the original mouseDown position. This caused
ctf.getItem() in Eclipse platform.ui's DnDInfo to return null when
the cursor had drifted off the narrow tab label, silently aborting
the drag.

Fix: use the queued mouseDown event's x/y and button instead of the
motion event values, matching the X11 behaviour.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/3192

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>