Preserve the target when scrolling w/ mousewheel in gutter

Removing the target of a mouseweel event messes up velocity scrolling
with the track pad, so it needs to be preserved until scrolling ceases.

src/editor-component.coffee

@@ -28,6 +28,7 @@ EditorComponent = React.createClass
   measuringScrollbars: true
   pendingVerticalScrollDelta: 0
   pendingHorizontalScrollDelta: 0
+  mouseWheelScreenRow: null
 
   render: ->
     {focused, fontSize, lineHeight, fontFamily, showIndentGuide} = @state
@@ -53,7 +54,7 @@ EditorComponent = React.createClass
       GutterComponent {
         ref: 'gutter', editor, renderedRowRange, maxLineNumberDigits,
         scrollTop, scrollHeight, lineHeight: lineHeightInPixels, fontSize, fontFamily,
-        @pendingChanges, onWidthChanged: @onGutterWidthChanged
+        @pendingChanges, onWidthChanged: @onGutterWidthChanged, @mouseWheelScreenRow
       }
 
       EditorScrollViewComponent {
@@ -325,7 +326,8 @@ EditorComponent = React.createClass
 
   onMouseWheel: (event) ->
     event.preventDefault()
-
+    screenRow = @screenRowForNode(event.target)
+    @mouseWheelScreenRow = screenRow if screenRow?
     animationFramePending = @pendingHorizontalScrollDelta isnt 0 or @pendingVerticalScrollDelta isnt 0
 
     # Only scroll in one direction at a time
@@ -343,6 +345,13 @@ EditorComponent = React.createClass
         @pendingVerticalScrollDelta = 0
         @pendingHorizontalScrollDelta = 0
 
+  screenRowForNode: (node) ->
+    while node isnt document
+      if screenRow = node.dataset.screenRow
+        return parseInt(screenRow)
+      node = node.parentNode
+    null
+
   onStylesheetsChanged: (stylesheet) ->
     @refreshScrollbars() if @containsScrollbarSelector(stylesheet)
 
@@ -408,6 +417,7 @@ EditorComponent = React.createClass
 
   onStoppedScrolling: ->
     @scrollingVertically = false
+    @mouseWheelScreenRow = null
     @requestUpdate()
 
   stopScrollingAfterDelay: null # created lazily

src/gutter-component.coffee

@@ -113,13 +113,15 @@ GutterComponent = React.createClass
     visibleLineNumberIds
 
   removeLineNumberNodes: (lineNumberIdsToPreserve) ->
+    {mouseWheelScreenRow} = @props
     node = @refs.lineNumbers.getDOMNode()
     for lineNumberId, lineNumberNode of @lineNumberNodesById when not lineNumberIdsToPreserve?.has(lineNumberId)
-      delete @lineNumberNodesById[lineNumberId]
       screenRow = @screenRowsByLineNumberId[lineNumberId]
-      delete @lineNumberIdsByScreenRow[screenRow] if @lineNumberIdsByScreenRow[screenRow] is lineNumberId
-      delete @screenRowsByLineNumberId[lineNumberId]
-      node.removeChild(lineNumberNode)
+      unless screenRow is mouseWheelScreenRow
+        delete @lineNumberNodesById[lineNumberId]
+        delete @lineNumberIdsByScreenRow[screenRow] if @lineNumberIdsByScreenRow[screenRow] is lineNumberId
+        delete @screenRowsByLineNumberId[lineNumberId]
+        node.removeChild(lineNumberNode)
 
   buildLineNumberHTML: (bufferRow, softWrapped, maxLineNumberDigits, screenRow) ->
     if screenRow?
@@ -129,7 +131,7 @@ GutterComponent = React.createClass
       style = "visibility: hidden;"
     innerHTML = @buildLineNumberInnerHTML(bufferRow, softWrapped, maxLineNumberDigits)
 
-    "<div class=\"line-number editor-colors\" style=\"#{style}\">#{innerHTML}</div>"
+    "<div class=\"line-number editor-colors\" style=\"#{style}\" data-screen-row=\"#{screenRow}\">#{innerHTML}</div>"
 
   buildLineNumberInnerHTML: (bufferRow, softWrapped, maxLineNumberDigits) ->
     if softWrapped