[GTK/Wayland] Fix DragDetect coordinates using mouseDown position

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 #3192

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

Author: vogella

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/widgets/Control.java

@@ -4143,14 +4143,14 @@ long gtk3_motion_notify_event (long widget, long event) {
 			int eventType = GDK.gdk_event_get_event_type(event);
 			if (eventType == GDK.GDK_3BUTTON_PRESS) return 0;
 
-			Point scaledEvent = new Point((int)eventX[0], (int) eventY[0]);
-
-			int [] eventButton = new int [1];
+			// Use the original mouseDown coordinates and button from the queued event,
+			// not the current motion event position (which is past the drag threshold
+			// and may be off the tab label, causing ctf.getItem() lookups to fail).
+			Event mouseDownEvent = dragDetectionQueue.getFirst();
 			int [] eventState = new int [1];
-			GDK.gdk_event_get_button(event, eventButton);
 			GDK.gdk_event_get_state(event, eventState);
 
-			if (sendDragEvent (eventButton[0], eventState[0], scaledEvent.x, scaledEvent.y, false)) {
+			if (sendDragEvent (mouseDownEvent.button, eventState[0], mouseDownEvent.x, mouseDownEvent.y, false)) {
 				return 1;
 			}
 		}