Prevent inserting segments in 'no-overlap' or 'compress' mode

This change prevents inserting new segments if the user
clicks on an existing segment, where the segment drag mode
is 'no-overlap' or 'compress'.

See #524

src/insert-segment-mouse-drag-handler.js

@@ -42,12 +42,17 @@ InsertSegmentMouseDragHandler.prototype._onMouseDown = function(mousePosX, segme
   this._segment = segment;
 
   if (this._segment) {
-    // The user has clicked within a segment. We want to prevent
-    // the segment from being dragged while the user inserts a new
-    // segment. So we temporarily make the segment non-draggable,
-    // and restore its draggable state in onMouseUp().
-    this._segmentIsDraggable = this._segment.draggable();
-    this._segment.draggable(false);
+    if (this._view.getSegmentDragMode() !== 'overlap') {
+      return;
+    }
+    else {
+      // The user has clicked within a segment. We want to prevent
+      // the segment from being dragged while the user inserts a new
+      // segment. So we temporarily make the segment non-draggable,
+      // and restore its draggable state in onMouseUp().
+      this._segmentIsDraggable = this._segment.draggable();
+      this._segment.draggable(false);
+    }
   }
 
   const time = this._view.pixelsToTime(mousePosX + this._view.getFrameOffset());
@@ -72,6 +77,10 @@ InsertSegmentMouseDragHandler.prototype._onMouseMove = function() {
 };
 
 InsertSegmentMouseDragHandler.prototype._onMouseUp = function() {
+  if (!this._insertSegment) {
+    return;
+  }
+
   if (this._insertSegmentShape) {
     this._insertSegmentShape.stopDrag();
     this._insertSegmentShape = null;

src/mouse-drag-handler.js

@@ -63,7 +63,7 @@ MouseDragHandler.prototype._mouseDown = function(event) {
 
   const marker = getMarkerObject(event.target);
 
-  if (marker && marker.attrs.draggable) {
+  if (marker) {
     // Avoid interfering with drag/drop of point and segment markers.
     if (marker.attrs.name === 'point-marker' ||
         marker.attrs.name === 'segment-marker') {

src/scroll-mouse-drag-handler.js

@@ -40,9 +40,15 @@ ScrollMouseDragHandler.prototype.isDragging = function() {
 };
 
 ScrollMouseDragHandler.prototype._onMouseDown = function(mousePosX, segment) {
-  this._segment = segment;
   this._seeking = false;
 
+  if (segment && !segment.attrs.draggable) {
+    this._segment = null;
+  }
+  else {
+    this._segment = segment;
+  }
+
   const playheadOffset = this._view.getPlayheadOffset();
 
   if (this._view.isSeekEnabled() &&