Add TextEditorPresenter::state.scrollingVertically

This is used by the view to defer measuring new lines until we stop
scrolling.

spec/text-editor-presenter-spec.coffee

@@ -63,6 +63,20 @@ describe "TextEditorPresenter", ->
         expectStateUpdate presenter, -> presenter.setScrollTop(50)
         expect(presenter.state.scrollTop).toBe 50
 
+    describe ".scrollingVertically", ->
+      it "is true for ::stoppedScrollingDelay milliseconds following a changes to ::scrollTop", ->
+        presenter = new TextEditorPresenter(model: editor, scrollTop: 10, stoppedScrollingDelay: 200)
+        expect(presenter.state.scrollingVertically).toBe false
+        expectStateUpdate presenter, -> presenter.setScrollTop(0)
+        expect(presenter.state.scrollingVertically).toBe true
+        advanceClock(100)
+        expect(presenter.state.scrollingVertically).toBe true
+        presenter.setScrollTop(10)
+        advanceClock(100)
+        expect(presenter.state.scrollingVertically).toBe true
+        expectStateUpdate presenter, -> advanceClock(100)
+        expect(presenter.state.scrollingVertically).toBe false
+
     describe ".content", ->
       describe ".scrollWidth", ->
         it "is initialized as the max of the clientWidth and the width of the longest line", ->

src/text-editor-presenter.coffee

@@ -6,8 +6,13 @@ module.exports =
 class TextEditorPresenter
   toggleCursorBlinkHandle: null
   startBlinkingCursorsAfterDelay: null
+  stoppedScrollingTimeoutId: null
+
+  constructor: (params) ->
+    {@model, @clientHeight, @clientWidth, @scrollTop, @scrollLeft} = params
+    {@lineHeight, @baseCharacterWidth, @lineOverdrawMargin, @backgroundColor} = params
+    {@cursorBlinkPeriod, @cursorBlinkResumeDelay, @stoppedScrollingDelay} = params
 
-  constructor: ({@model, @clientHeight, @clientWidth, @scrollTop, @scrollLeft, @lineHeight, @baseCharacterWidth, @lineOverdrawMargin, @cursorBlinkPeriod, @cursorBlinkResumeDelay, @backgroundColor}) ->
     @disposables = new CompositeDisposable
     @emitter = new Emitter
     @charWidthsByScope = {}
@@ -43,6 +48,7 @@ class TextEditorPresenter
 
   buildState: ->
     @state =
+      scrollingVertically: false
       content:
         blinkCursorsOff: false
         lines: {}
@@ -321,12 +327,25 @@ class TextEditorPresenter
   getCursorBlinkResumeDelay: -> @cursorBlinkResumeDelay
 
   setScrollTop: (@scrollTop) ->
+    @didStartScrolling()
     @updateVerticalScrollState()
     @updateLinesState()
     @updateCursorsState()
     @updateHighlightsState()
     @updateLineNumbersState()
 
+  didStartScrolling: ->
+    if @stoppedScrollingTimeoutId?
+      clearTimeout(@stoppedScrollingTimeoutId)
+      @stoppedScrollingTimeoutId = null
+    @stoppedScrollingTimeoutId = setTimeout(@didStopScrolling.bind(this), @stoppedScrollingDelay)
+    @state.scrollingVertically = true
+    @emitter.emit 'did-update-state'
+
+  didStopScrolling: ->
+    @state.scrollingVertically = false
+    @emitter.emit 'did-update-state'
+
   getScrollTop: -> @scrollTop
 
   setScrollLeft: (@scrollLeft) ->