Eliminate overlayer by preserving lines during mousewheel events

Previously, the overlayer served as a permanent target for mousewheel
events that would never be removed by scrolling. This is because the
velocity scrolling effect on a trackpad is implemented by repeating
events on the original mousewheel event target. If this target is
removed, the events stop repeating and the velocity effect is ruined.
Now we refrain from removing any lines until mousewheel events stop
flowing.

src/editor-component.coffee

@@ -1,5 +1,6 @@
 {React, div, span} = require 'reactionary'
 {$$} = require 'space-pen'
+{debounce} = require 'underscore-plus'
 
 InputComponent = require './input-component'
 SelectionComponent = require './selection-component'
@@ -37,20 +38,13 @@ EditorCompont = React.createClass
     WebkitTransform = "translateY(#{-editor.getScrollTop()}px)"
 
     div className: 'scrollable-content', style: {height, WebkitTransform},
-      @renderOverlayer()
+      @renderCursors()
       @renderVisibleLines()
       @renderUnderlayer()
 
-  renderOverlayer: ->
-    {editor} = @props
-
-    div className: 'overlayer',
-      for selection in editor.getSelections() when editor.selectionIntersectsVisibleRowRange(selection)
-        CursorComponent(cursor: selection.cursor)
-
   renderVisibleLines: ->
     {editor} = @props
-    [startRow, endRow] = editor.getVisibleRowRange()
+    [startRow, endRow] = @getVisibleRowRange()
     lineHeightInPixels = editor.getLineHeight()
     precedingHeight = startRow * lineHeightInPixels
     followingHeight = (editor.getScreenLineCount() - endRow) * lineHeightInPixels
@@ -62,6 +56,12 @@ EditorCompont = React.createClass
       div className: 'spacer', key: 'bottom-spacer', style: {height: followingHeight}
     ]
 
+  renderCursors: ->
+    {editor} = @props
+
+    for selection in editor.getSelections() when editor.selectionIntersectsVisibleRowRange(selection)
+      CursorComponent(cursor: selection.cursor)
+
   renderUnderlayer: ->
     {editor} = @props
 
@@ -69,6 +69,13 @@ EditorCompont = React.createClass
       for selection in editor.getSelections() when editor.selectionIntersectsVisibleRowRange(selection)
         SelectionComponent({selection})
 
+  getVisibleRowRange: ->
+    visibleRowRange = @props.editor.getVisibleRowRange()
+    if @visibleRowOverrides?
+      visibleRowRange[0] = Math.min(visibleRowRange[0], @visibleRowOverrides[0])
+      visibleRowRange[1] = Math.max(visibleRowRange[1], @visibleRowOverrides[1])
+    visibleRowRange
+
   getInitialState: -> {}
 
   getDefaultProps: -> updateSync: true
@@ -246,9 +253,22 @@ EditorCompont = React.createClass
         @pendingScrollTop = null
 
   onMousewheel: (event) ->
+    # To preserve velocity scrolling, delay removal of the event's target until
+    # after mousewheel events stop being fired. Removing the target before then
+    # will cause scrolling to stop suddenly.
+    @visibleRowOverrides = @getVisibleRowRange()
+    @clearVisibleRowOverridesAfterDelay ?= debounce(@clearVisibleRowOverrides, 40)
+    @clearVisibleRowOverridesAfterDelay()
+
     @refs.verticalScrollbar.getDOMNode().scrollTop -= event.wheelDeltaY
     event.preventDefault()
 
+  clearVisibleRowOverrides: ->
+    @visibleRowOverrides = null
+    @forceUpdate()
+
+  clearVisibleRowOverridesAfterDelay: null
+
   onOverflowChanged: ->
     @props.editor.setHeight(@refs.scrollView.getDOMNode().clientHeight)
 
@@ -300,7 +320,7 @@ EditorCompont = React.createClass
     {lineHeightInPixels, charWidth}
 
   measureNewLines: ->
-    [visibleStartRow, visibleEndRow] = @props.editor.getVisibleRowRange()
+    [visibleStartRow, visibleEndRow] = @getVisibleRowRange()
     linesNode = @refs.lines.getDOMNode()
 
     for tokenizedLine, i in @props.editor.linesForScreenRows(visibleStartRow, visibleEndRow - 1)