(js) Ignore mouse events in scrollbars of Month

Firefox is bubbling the mouse down event on scrollbars so it has to be
ignored for proper drag'n'drop.

Fixes #3990.

UI/WebServerResources/js/Scheduler/sgDraggableCalendarBlock.directive.js

@@ -81,10 +81,20 @@
       }
 
       function onDragDetect(ev) {
-        var dragMode, pointerHandler;
+        var dragMode, pointerHandler, hasVerticalScrollbar, rect;
 
         ev.stopPropagation();
 
+        hasVerticalScrollbar = ev.target.scrollHeight > ev.target.clientHeight + 1;
+
+        if (hasVerticalScrollbar) {
+          // Check if mouse click is inside scrollbar
+          rect = ev.target.getBoundingClientRect();
+          scrollableZone = rect.left + rect.width - 18;
+          if (ev.pageX > scrollableZone)
+            return;
+        }
+
         dragMode = 'move-event';
 
         if (scope.block && scope.block.component) {