Ignore 'scroll' events when an update is pending

This prevents feedback loops where we handle stale 'scroll' events
for scrolls requested in the model layer. It prevents jitter when
autoscrolling with the cursor.

spec/editor-component-spec.coffee

@@ -724,11 +724,11 @@ describe "EditorComponent", ->
       # Add decorations that are out of range
       marker2 = editor.displayBuffer.markBufferRange([[9, 0], [9, 0]])
       editor.addDecorationForMarker(marker2, type: ['gutter', 'line'], class: 'b')
+      runSetImmediateCallbacks()
 
       # Scroll decorations into view
       verticalScrollbarNode.scrollTop = 2.5 * lineHeightInPixels
       verticalScrollbarNode.dispatchEvent(new UIEvent('scroll'))
-      runSetImmediateCallbacks()
       expect(lineAndLineNumberHaveClass(9, 'b')).toBe true
 
       # Fold a line to move the decorations

src/editor-component.coffee

@@ -529,7 +529,7 @@ EditorComponent = React.createClass
   onVerticalScroll: (scrollTop) ->
     {editor} = @props
 
-    return if scrollTop is editor.getScrollTop()
+    return if @updateRequested or scrollTop is editor.getScrollTop()
 
     animationFramePending = @pendingScrollTop?
     @pendingScrollTop = scrollTop
@@ -542,7 +542,7 @@ EditorComponent = React.createClass
   onHorizontalScroll: (scrollLeft) ->
     {editor} = @props
 
-    return if scrollLeft is editor.getScrollLeft()
+    return if @updateRequested or scrollLeft is editor.getScrollLeft()
 
     animationFramePending = @pendingScrollLeft?
     @pendingScrollLeft = scrollLeft