Fix dragging regions to/from drop zone (#6137).

The previous code assumed that dragging up from the drop zone into the last
track is always valid.  This is not true when the type of the dragged region(s)
do not match that route, which resulted in a crash and/or bizarre behaviour.

This took entirely too long to figure out, there are some real nightmares in
the region drag code...

1 files changed, 17 insertions, 17 deletions

diff --git a/gtk2_ardour/editor_drag.cc b/gtk2_ardour/editor_drag.cc
index e8fdd33236..a33b0fe01a 100644
--- a/gtk2_ardour/editor_drag.cc
+++ b/gtk2_ardour/editor_drag.cc
@@ -652,30 +652,27 @@ RegionMotionDrag::y_movement_allowed (int delta_track, double delta_layer) const
 		int n = i->time_axis_view + delta_track;
 		if (i->time_axis_view < 0) {
 			/* already in the drop zone */
-			if (delta_track > 0) {
+			if (delta_track >= 0) {
 				/* downward motion - might be OK if others are still not in the dropzone,
 				   so check at the end of the loop if that is the case.
 				 */
 				continue;
-			} 
-			/* upward motion, and this view is currently in the drop zone. That's fine, so 
-			   we have to set all_in_drop_zone correctly to avoid blocking upward motion.
-			*/
-			all_in_drop_zone = false; 
-			continue;
-		} else {
-
-			all_in_drop_zone = false;
-			
-			if (n < 0) {
-				/* off the top */
-				return false;
 			}
+
+			/* upward motion - set n to the track we would end up in if motion
+			   is successful, and check validity below. */
+			n = _time_axis_views.size() + delta_track;
 		}
 
-		if (n >= int (_time_axis_views.size())) {
+		if (n < 0) {
+			/* off the top */
+			return false;
+		} else if (n >= int (_time_axis_views.size())) {
 			/* downward motion into drop zone. That's fine. */
 			continue;
+		} else {
+			/* target is not in the drop zone */
+			all_in_drop_zone = false;
 		}
 		
 		RouteTimeAxisView const * to = dynamic_cast<RouteTimeAxisView const *> (_time_axis_views[n]);
@@ -749,8 +746,11 @@ RegionMotionDrag::motion (GdkEvent* event, bool first_move)
 		/* Work out the change in y */
 
 		if (_last_pointer_time_axis_view < 0) {
-			/* we can only move up */
-			delta_time_axis_view = -1;
+			/* We can only move up, set delta to move up the correct number of
+			   tracks from the bottom.  This is necessary because steps may be
+			   skipped if the lowest track is not a valid target, so e.g. delta
+			   -2 may be the first valid value. */
+			delta_time_axis_view = current_pointer_time_axis_view - _time_axis_views.size();
 		} else {
 			delta_time_axis_view = current_pointer_time_axis_view - _last_pointer_time_axis_view;
 		}