Preserve tiles that are the target of mousewheel events

This allows velocity-style mouse wheel scrolling to continue once the
tile has been scrolled off screen.

src/editor-tile-component.coffee

@@ -12,6 +12,7 @@ class EditorTileComponent
   lineHeightInPixels: null
   lineWidths: null
   backgroundColor: null
+  preserved: false
 
   constructor: (@presenter) ->
     @lineNodesByLineId = {}
@@ -22,6 +23,7 @@ class EditorTileComponent
     @cursorNodesById = {}
 
     @domNode = document.createElement('div')
+    @domNode.dataset.tile = true
     @domNode.style.position = 'absolute'
     @domNode.style.overflow = 'hidden'
 
@@ -43,6 +45,15 @@ class EditorTileComponent
     # @clearScreenRowCaches() if newProps.lineHeightInPixels isnt @props.lineHeightInPixels
     # @updateCursors()
 
+  preserve: ->
+    return if @preserved
+    @domNode.style.visibility = 'hidden'
+    @preserved = true
+
+  revive: (@presenter) ->
+    @domNode.style.visibility = ''
+    @visible = true
+
   updateTransform: ->
     {left, top} = @presenter
     unless left is @left and top is @top

src/gutter-component.coffee

@@ -24,6 +24,7 @@ GutterComponent = React.createClass
 
   componentDidMount: ->
     @appendDummyTile()
+    @getDOMNode().addEventListener 'mousewheel', @onMouseWheel
 
   componentDidUpdate: (oldProps) ->
     if @props.performedInitialMeasurement
@@ -36,12 +37,16 @@ GutterComponent = React.createClass
 
     for tileStartRow, tileComponent of @tileComponentsByStartRow
       unless gutterPresenter.tiles[tileStartRow]?
-        domNode.removeChild(tileComponent.domNode)
-        delete @tileComponentsByStartRow[tileStartRow]
+        if tileComponent.domNode is @preservedTileNode
+          tileComponent.preserve()
+        else
+          domNode.removeChild(tileComponent.domNode)
+          delete @tileComponentsByStartRow[tileStartRow]
 
     for tileStartRow, tilePresenter of gutterPresenter.tiles
       if tileComponent = @tileComponentsByStartRow[tileStartRow]
         tileComponent = @tileComponentsByStartRow[tileStartRow]
+        tileComponent.revive(tilePresenter) if tileComponent.preserved
         tileComponent.update()
       else
         tileComponent = new GutterTileComponent(tilePresenter)
@@ -77,3 +82,8 @@ GutterComponent = React.createClass
         editor.unfoldBufferRow(bufferRow)
       else
         editor.foldBufferRow(bufferRow)
+
+  onMouseWheel: (event) ->
+    node = event.target
+    node = node.parentNode until not node? or node.dataset.tile
+    @preservedTileNode = node

src/gutter-tile-component.coffee

@@ -3,12 +3,15 @@ WrapperDiv = document.createElement('div')
 
 module.exports =
 class GutterTileComponent
+  preserved: false
+
   constructor: (@presenter) ->
     @lineNumberNodesById = {}
     @screenRowsByLineNumberId = {}
     @lineNumberDecorationsByLineNumberId = {}
 
     @domNode = document.createElement('div')
+    @domNode.dataset.tile = true
     @domNode.style.overflow = 'hidden'
     if @presenter.dummy
       @domNode.style.visibility = 'hidden'
@@ -30,6 +33,15 @@ class GutterTileComponent
     @updateBackgroundColor()
     @updateLineNumbers()
 
+  preserve: ->
+    return if @preserved
+    @domNode.style.visibility = 'hidden'
+    @preserved = true
+
+  revive: (@presenter) ->
+    @domNode.style.visibility = ''
+    @visible = true
+
   updateTransform: ->
     {top} = @presenter
     unless top is @top

src/lines-component.coffee

@@ -18,6 +18,7 @@ LinesComponent = React.createClass
   displayName: 'LinesComponent'
 
   tileSize: 5
+  preservedTileNode: null
 
   render: ->
     div className: 'lines'
@@ -26,6 +27,9 @@ LinesComponent = React.createClass
     @measuredLines = new WeakSet
     @tileComponentsByStartRow = {}
 
+  componentDidMount: ->
+    @getDOMNode().addEventListener 'mousewheel', @onMouseWheel
+
   shouldComponentUpdate: (newProps) ->
     return true
 
@@ -80,12 +84,16 @@ LinesComponent = React.createClass
 
     for tileStartRow, tileComponent of @tileComponentsByStartRow
       unless contentPresenter.tiles[tileStartRow]?
-        domNode.removeChild(tileComponent.domNode)
-        delete @tileComponentsByStartRow[tileStartRow]
+        if tileComponent.domNode is @preservedTileNode
+          tileComponent.preserve()
+        else
+          domNode.removeChild(tileComponent.domNode)
+          delete @tileComponentsByStartRow[tileStartRow]
 
     for tileStartRow, tilePresenter of contentPresenter.tiles
       if tileComponent = @tileComponentsByStartRow[tileStartRow]
         tileComponent = @tileComponentsByStartRow[tileStartRow]
+        tileComponent.revive(tilePresenter) if tileComponent.preserved
         tileComponent.update()
       else
         tileComponent = new EditorTileComponent(tilePresenter)
@@ -153,3 +161,8 @@ LinesComponent = React.createClass
   clearScopedCharWidths: ->
     @measuredLines.clear()
     @props.editor.clearScopedCharWidths()
+
+  onMouseWheel: (event) ->
+    node = event.target
+    node = node.parentNode until node.dataset.tile
+    @preservedTileNode = node