Make Viewport::DragToScrollListener more robust and able to cope with the deletion of the original mouseDown event component

modules/juce_gui_basics/layout/juce_Viewport.cpp

@@ -217,6 +217,7 @@ struct Viewport::DragToScrollListener   : private MouseListener,
     ~DragToScrollListener()
     {
         viewport.contentHolder.removeMouseListener (this);
+        Desktop::getInstance().removeGlobalMouseListener (this);
     }
 
     void positionChanged (ViewportDragPosition&, double) override
@@ -227,14 +228,23 @@ struct Viewport::DragToScrollListener   : private MouseListener,
 
     void mouseDown (const MouseEvent&) override
     {
-        offsetX.setPosition (offsetX.getPosition());
-        offsetY.setPosition (offsetY.getPosition());
-        ++numTouches;
+        if (! isGlobalMouseListener)
+        {
+            offsetX.setPosition (offsetX.getPosition());
+            offsetY.setPosition (offsetY.getPosition());
+
+            // switch to a global mouse listener so we still receive mouseUp events
+            // if the original event component is deleted
+            viewport.contentHolder.removeMouseListener (this);
+            Desktop::getInstance().addGlobalMouseListener (this);
+
+            isGlobalMouseListener = true;
+        }
     }
 
     void mouseDrag (const MouseEvent& e) override
     {
-        if (numTouches == 1 && ! doesMouseEventComponentBlockViewportDrag (e.eventComponent))
+        if (Desktop::getInstance().getNumDraggingMouseSources() == 1 && ! doesMouseEventComponentBlockViewportDrag (e.eventComponent))
         {
             auto totalOffset = e.getOffsetFromDragStart().toFloat();
 
@@ -259,13 +269,20 @@ struct Viewport::DragToScrollListener   : private MouseListener,
 
     void mouseUp (const MouseEvent&) override
     {
-        if (--numTouches <= 0)
-        {
-            offsetX.endDrag();
-            offsetY.endDrag();
-            isDragging = false;
-            numTouches = 0;
-        }
+        if (isGlobalMouseListener && Desktop::getInstance().getNumDraggingMouseSources() == 0)
+            endDragAndClearGlobalMouseListener();
+    }
+
+    void endDragAndClearGlobalMouseListener()
+    {
+        offsetX.endDrag();
+        offsetY.endDrag();
+        isDragging = false;
+
+        viewport.contentHolder.addMouseListener (this, true);
+        Desktop::getInstance().removeGlobalMouseListener (this);
+
+        isGlobalMouseListener = false;
     }
 
     bool doesMouseEventComponentBlockViewportDrag (const Component* eventComp)
@@ -280,8 +297,8 @@ struct Viewport::DragToScrollListener   : private MouseListener,
     Viewport& viewport;
     ViewportDragPosition offsetX, offsetY;
     Point<int> originalViewPos;
-    int numTouches = 0;
     bool isDragging = false;
+    bool isGlobalMouseListener = false;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (DragToScrollListener)
 };