Only prevent default on mousewheel events if editor actually scrolls

This prevents mini editors from capturing scroll events.

spec/editor-component-spec.coffee

@@ -1687,6 +1687,42 @@ describe "EditorComponent", ->
 
         expect(componentNode.contains(lineNumberNode)).toBe true
 
+    it "only prevents the default action of the mousewheel event if it actually lead to scrolling", ->
+      spyOn(WheelEvent::, 'preventDefault').andCallThrough()
+
+      wrapperNode.style.height = 4.5 * lineHeightInPixels + 'px'
+      wrapperNode.style.width = 20 * charWidth + 'px'
+      component.measureHeightAndWidth()
+      runSetImmediateCallbacks()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: 0, wheelDeltaY: 50))
+      expect(editor.getScrollTop()).toBe 0
+      expect(WheelEvent::preventDefault).not.toHaveBeenCalled()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: 0, wheelDeltaY: -3000))
+      runSetImmediateCallbacks()
+      expect(editor.getScrollTop()).toBe editor.getScrollHeight() - editor.getHeight() + 15
+      expect(WheelEvent::preventDefault).toHaveBeenCalled()
+      WheelEvent::preventDefault.reset()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: 0, wheelDeltaY: -30))
+      expect(editor.getScrollTop()).toBe editor.getScrollHeight() - editor.getHeight() + 15
+      expect(WheelEvent::preventDefault).not.toHaveBeenCalled()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: 50, wheelDeltaY: 0))
+      expect(editor.getScrollLeft()).toBe 0
+      expect(WheelEvent::preventDefault).not.toHaveBeenCalled()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: -3000, wheelDeltaY: 0))
+      runSetImmediateCallbacks()
+      expect(editor.getScrollLeft()).toBe editor.getScrollWidth() - editor.getWidth() + 15
+      expect(WheelEvent::preventDefault).toHaveBeenCalled()
+      WheelEvent::preventDefault.reset()
+
+      componentNode.dispatchEvent(new WheelEvent('mousewheel', wheelDeltaX: -30, wheelDeltaY: 0))
+      expect(editor.getScrollLeft()).toBe editor.getScrollWidth() - editor.getWidth() + 15
+      expect(WheelEvent::preventDefault).not.toHaveBeenCalled()
+
   describe "input events", ->
     inputNode = null
 

src/editor-component.coffee

@@ -567,20 +567,23 @@ EditorComponent = React.createClass
         @pendingScrollLeft = null
 
   onMouseWheel: (event) ->
-    event.preventDefault()
     {editor} = @props
 
     # Only scroll in one direction at a time
     {wheelDeltaX, wheelDeltaY} = event
     if Math.abs(wheelDeltaX) > Math.abs(wheelDeltaY)
       # Scrolling horizontally
-      editor.setScrollLeft(editor.getScrollLeft() - Math.round(wheelDeltaX * @scrollSensitivity))
+      previousScrollLeft = editor.getScrollLeft()
+      editor.setScrollLeft(previousScrollLeft - Math.round(wheelDeltaX * @scrollSensitivity))
+      event.preventDefault() unless previousScrollLeft is editor.getScrollLeft()
     else
       # Scrolling vertically
       @mouseWheelScreenRow = @screenRowForNode(event.target)
       @clearMouseWheelScreenRowAfterDelay ?= debounce(@clearMouseWheelScreenRow, @mouseWheelScreenRowClearDelay)
       @clearMouseWheelScreenRowAfterDelay()
-      editor.setScrollTop(editor.getScrollTop() - Math.round(wheelDeltaY * @scrollSensitivity))
+      previousScrollTop = editor.getScrollTop()
+      editor.setScrollTop(previousScrollTop - Math.round(wheelDeltaY * @scrollSensitivity))
+      event.preventDefault() unless previousScrollTop is editor.getScrollTop()
 
   onScrollViewScroll: ->
     if @isMounted()