Clear the mousewheelScreenRow even if the event does not cause scrolling

If a mousewheel event is triggered when the editor can't be scrolled,
we still want to clear the mouseWheelScreenRow. This is typically done
when we stop scrolling, but if we never start scrolling it will never
happen. This commit adds another timeout to cover that case.

spec/editor-component-spec.coffee

@@ -790,6 +790,22 @@ describe "EditorComponent", ->
 
         expect(component.mouseWheelScreenRow).toBe null
 
+      it "clears the mouseWheelScreenRow after a delay even if the event does not cause scrolling", ->
+        spyOn(_._, 'now').andCallFake -> window.now # Ensure _.debounce is based on our fake spec timeline
+
+        expect(editor.getScrollTop()).toBe 0
+
+        lineNode = node.querySelector('.line')
+        wheelEvent = new WheelEvent('mousewheel', wheelDeltaX: 0, wheelDeltaY: 10)
+        Object.defineProperty(wheelEvent, 'target', get: -> lineNode)
+        node.dispatchEvent(wheelEvent)
+
+        expect(editor.getScrollTop()).toBe 0
+
+        expect(component.mouseWheelScreenRow).toBe 0
+        advanceClock(component.mouseWheelScreenRowClearDelay)
+        expect(component.mouseWheelScreenRow).toBe null
+
       it "does not preserve the line if it is on screen", ->
         expect(node.querySelectorAll('.line-number').length).toBe 14 # dummy line
         lineNodes = node.querySelectorAll('.line')

src/editor-component.coffee

@@ -29,6 +29,7 @@ EditorComponent = React.createClass
   pendingVerticalScrollDelta: 0
   pendingHorizontalScrollDelta: 0
   mouseWheelScreenRow: null
+  mouseWheelScreenRowClearDelay: 150
 
   render: ->
     {focused, fontSize, lineHeight, fontFamily, showIndentGuide, showInvisibles, visible} = @state
@@ -370,6 +371,8 @@ EditorComponent = React.createClass
       # Scrolling vertically
       @pendingVerticalScrollDelta -= wheelDeltaY
       @mouseWheelScreenRow = @screenRowForNode(event.target)
+      @clearMouseWheelScreenRowAfterDelay ?= debounce(@clearMouseWheelScreenRow, @mouseWheelScreenRowClearDelay)
+      @clearMouseWheelScreenRowAfterDelay()
 
     unless animationFramePending
       requestAnimationFrame =>
@@ -379,6 +382,13 @@ EditorComponent = React.createClass
         @pendingVerticalScrollDelta = 0
         @pendingHorizontalScrollDelta = 0
 
+  clearMouseWheelScreenRow: ->
+    if @mouseWheelScreenRow?
+      @mouseWheelScreenRow = null
+      @requestUpdate()
+
+  clearMouseWheelScreenRowAfterDelay: null # created lazily
+
   screenRowForNode: (node) ->
     while node isnt document
       if screenRow = node.dataset.screenRow