Handle line and line number preservation in presenter

The target of mousewheel events needs to be preserved when scrolling.
It used to be dealt with in the view, but now we can do it in the
presenter for a simpler view implementation.

src/gutter-component.coffee

@@ -69,7 +69,7 @@ GutterComponent = React.createClass
     @dummyLineNumberNode.innerHTML = @buildLineNumberInnerHTML(0, false)
 
   updateLineNumbers: ->
-    {presenter, mouseWheelScreenRow} = @props
+    {presenter} = @props
     @oldState ?= {lineNumbers: {}}
     newState = presenter.state.gutter
     newLineNumberIds = null
@@ -96,7 +96,7 @@ GutterComponent = React.createClass
         node.appendChild(lineNumberNode)
 
     for id, lineNumberState of @oldState.lineNumbers
-      unless newState.lineNumbers.hasOwnProperty(id) or lineNumberState.screenRow is mouseWheelScreenRow
+      unless newState.lineNumbers.hasOwnProperty(id)
         @lineNumberNodesById[id].remove()
         delete @lineNumberNodesById[id]
         delete @oldState.lineNumbers[id]

src/lines-component.coffee

@@ -92,10 +92,10 @@ LinesComponent = React.createClass
     delete @oldState.content.lines[id]
 
   updateLineNodes: ->
-    {presenter, mouseWheelScreenRow} = @props
+    {presenter} = @props
 
     for id of @oldState.content.lines
-      unless @newState.content.lines.hasOwnProperty(id) or mouseWheelScreenRow is @screenRowsByLineId[id]
+      unless @newState.content.lines.hasOwnProperty(id)
         @removeLineNode(id)
 
     newLineIds = null

src/text-editor-component.coffee

@@ -31,8 +31,6 @@ TextEditorComponent = React.createClass
   updateRequestedWhilePaused: false
   cursorMoved: false
   selectionChanged: false
-  mouseWheelScreenRow: null
-  mouseWheelScreenRowClearDelay: 150
   scrollSensitivity: 0.4
   heightAndWidthMeasurementRequested: false
   inputEnabled: true
@@ -66,8 +64,6 @@ TextEditorComponent = React.createClass
       horizontallyScrollable = editor.horizontallyScrollable()
       hiddenInputStyle = @getHiddenInputPosition()
       hiddenInputStyle.WebkitTransform = 'translateZ(0)' if @useHardwareAcceleration
-      if @mouseWheelScreenRow? and not (renderedStartRow <= @mouseWheelScreenRow < renderedEndRow)
-        mouseWheelScreenRow = @mouseWheelScreenRow
 
       style.height = scrollHeight if @autoHeight
 
@@ -82,7 +78,7 @@ TextEditorComponent = React.createClass
       if @gutterVisible
         GutterComponent {
           ref: 'gutter', onMouseDown: @onGutterMouseDown,
-          @presenter, editor, mouseWheelScreenRow, @useHardwareAcceleration
+          @presenter, editor, @useHardwareAcceleration
         }
 
       div ref: 'scrollView', className: 'scroll-view',
@@ -92,8 +88,7 @@ TextEditorComponent = React.createClass
           style: hiddenInputStyle
 
         LinesComponent {
-          ref: 'lines', @presenter, editor, hostElement, @useHardwareAcceleration, useShadowDOM,
-          mouseWheelScreenRow, visible
+           ref: 'lines', @presenter, editor, hostElement, @useHardwareAcceleration, useShadowDOM, visible
         }
 
         ScrollbarComponent
@@ -433,9 +428,7 @@ TextEditorComponent = React.createClass
       event.preventDefault() unless previousScrollLeft is editor.getScrollLeft()
     else
       # Scrolling vertically
-      @mouseWheelScreenRow = @screenRowForNode(event.target)
-      @clearMouseWheelScreenRowAfterDelay ?= debounce(@clearMouseWheelScreenRow, @mouseWheelScreenRowClearDelay)
-      @clearMouseWheelScreenRowAfterDelay()
+      @presenter.setMouseWheelScreenRow(@screenRowForNode(event.target))
       previousScrollTop = editor.getScrollTop()
       editor.setScrollTop(previousScrollTop - Math.round(wheelDeltaY * @scrollSensitivity))
       event.preventDefault() unless previousScrollTop is editor.getScrollTop()
@@ -846,13 +839,6 @@ TextEditorComponent = React.createClass
     horizontalNode.style.display = originalHorizontalDisplayValue
     cornerNode.style.display = originalCornerDisplayValue
 
-  clearMouseWheelScreenRow: ->
-    if @mouseWheelScreenRow?
-      @mouseWheelScreenRow = null
-      @requestUpdate()
-
-  clearMouseWheelScreenRowAfterDelay: null # created lazily
-
   consolidateSelections: (e) ->
     e.abortKeyBinding() unless @props.editor.consolidateSelections()
 

src/text-editor-presenter.coffee

@@ -7,6 +7,7 @@ class TextEditorPresenter
   toggleCursorBlinkHandle: null
   startBlinkingCursorsAfterDelay: null
   stoppedScrollingTimeoutId: null
+  mouseWheelScreenRow: null
 
   constructor: (params) ->
     {@model, @clientHeight, @clientWidth, @scrollTop, @scrollLeft} = params