counting dragLeave/Enter events is not reliable

lib/ace/mouse/dragdrop_handler.js

@@ -60,7 +60,7 @@ function DragdropHandler(mouseHandler) {
     var mouseTarget = editor.container;
     var dragSelectionMarker, x, y;
     var timerId, range;
-    var dragCursor, counter = 0;
+    var dragCursor, dragLeaveT = null;
     var dragOperation;
 
     this.onDragStart = function(e) {
@@ -105,9 +105,11 @@ function DragdropHandler(mouseHandler) {
     this.onDragEnter = function(e) {
         if (editor.getReadOnly() || !canAccept(e.dataTransfer))
             return;
+        if (dragLeaveT) {
+            dragLeaveT = null;
+        }
         if (!dragSelectionMarker)
             addDragMarker();
-        counter++;
         // dataTransfer object does not save dropEffect across events on IE, so we store it in dragOperation
         e.dataTransfer.dropEffect = dragOperation = getDropEffect(e);
         return event.preventDefault(e);
@@ -116,10 +118,12 @@ function DragdropHandler(mouseHandler) {
     this.onDragOver = function(e) {
         if (editor.getReadOnly() || !canAccept(e.dataTransfer))
             return;
+        if (dragLeaveT) {
+            dragLeaveT = null;
+        }
         // Opera doesn't trigger dragenter event on drag start
         if (!dragSelectionMarker) {
             addDragMarker();
-            counter++;
         }
         if (onMouseMoveTimer !== null)
             onMouseMoveTimer = null;
@@ -131,13 +135,23 @@ function DragdropHandler(mouseHandler) {
     };
 
     this.onDragLeave = function(e) {
-        counter--;
-        if (counter <= 0 && dragSelectionMarker) {
-            clearDragMarker();
-            dragOperation = null;
-            return event.preventDefault(e);
+        if (!dragLeaveT && dragSelectionMarker) {
+            dragLeaveT = Date.now();
+            dragLeaveTimer = setTimeout(onDragLeaveInternal, 100);
         }
     };
+    var dragLeaveTimer = null;
+    function onDragLeaveInternal() {
+        dragLeaveTimer = null;
+        if (!dragLeaveT) return;
+        var dt = Date.now() - dragLeaveT;
+        if (dt <= 80) {
+            dragLeaveTimer = setTimeout(onDragLeaveInternal, 100 - dt);
+        } else {
+            clearDragMarker();
+            dragLeaveT = dragOperation = null;
+        }
+    }
 
     this.onDrop = function(e) {
         if (!dragSelectionMarker)
@@ -217,7 +231,7 @@ function DragdropHandler(mouseHandler) {
         editor.clearSelection();
         clearInterval(timerId);
         timerId = setInterval(onDragInterval, 20);
-        counter = 0;
+        dragLeaveT = null;
         event.addListener(document, "mousemove", onMouseMove);
     }
 
@@ -229,7 +243,7 @@ function DragdropHandler(mouseHandler) {
         editor.selection.fromOrientedRange(range);
         editor.$blockScrolling -= 1;
         range = null;
-        counter = 0;
+        dragLeaveT = null;
         event.removeListener(document, "mousemove", onMouseMove);
     }