Add TextEditorPresenter::state.mousewheelScreenRow

spec/text-editor-presenter-spec.coffee

@@ -77,6 +77,41 @@ describe "TextEditorPresenter", ->
         expectStateUpdate presenter, -> advanceClock(100)
         expect(presenter.state.scrollingVertically).toBe false
 
+    describe ".mousewheelScreenRow", ->
+      it "reflects the most recently assigned ::mousewheelScreenRow while .scrollingVertically is true", ->
+        presenter = new TextEditorPresenter(model: editor, scrollTop: 10, stoppedScrollingDelay: 200)
+        presenter.setMousewheelScreenRow(3)
+        expect(presenter.state.scrollingVertically).toBe false
+        expect(presenter.state.mousewheelScreenRow).toBeNull()
+
+        expectStateUpdate presenter, -> presenter.setScrollTop(0)
+        expect(presenter.state.scrollingVertically).toBe true
+        expect(presenter.state.mousewheelScreenRow).toBe 3
+
+        presenter.setMousewheelScreenRow(5)
+        expect(presenter.state.scrollingVertically).toBe true
+        expect(presenter.state.mousewheelScreenRow).toBe 5
+
+        advanceClock(100)
+        expect(presenter.state.scrollingVertically).toBe true
+        expect(presenter.state.mousewheelScreenRow).toBe 5
+
+        # should wait 200ms after the last scroll to clear
+        presenter.setScrollTop(10)
+
+        advanceClock(100) # so not yet...
+        expect(presenter.state.scrollingVertically).toBe true
+        expect(presenter.state.mousewheelScreenRow).toBe 5
+
+        expectStateUpdate presenter, -> advanceClock(100) # clear now
+        expect(presenter.state.scrollingVertically).toBe false
+        expect(presenter.state.mousewheelScreenRow).toBeNull()
+
+        # should be cleared even when we scroll again
+        expectStateUpdate presenter, -> presenter.setScrollTop(20)
+        expect(presenter.state.scrollingVertically).toBe true
+        expect(presenter.state.mousewheelScreenRow).toBeNull()
+
     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

@@ -50,6 +50,7 @@ class TextEditorPresenter
   buildState: ->
     @state =
       scrollingVertically: false
+      mousewheelScreenRow: null
       content:
         blinkCursorsOff: false
         lines: {}
@@ -360,10 +361,13 @@ class TextEditorPresenter
       @stoppedScrollingTimeoutId = null
     @stoppedScrollingTimeoutId = setTimeout(@didStopScrolling.bind(this), @stoppedScrollingDelay)
     @state.scrollingVertically = true
+    @state.mousewheelScreenRow = @getMousewheelScreenRow()
     @emitter.emit 'did-update-state'
 
   didStopScrolling: ->
     @state.scrollingVertically = false
+    @state.mousewheelScreenRow = null
+    @mousewheelScreenRow = null
     @emitter.emit 'did-update-state'
 
   getScrollTop: -> @scrollTop
@@ -413,6 +417,11 @@ class TextEditorPresenter
 
   getLineHeight: -> @lineHeight
 
+  setMousewheelScreenRow: (@mousewheelScreenRow) ->
+    @state.mousewheelScreenRow = @mousewheelScreenRow if @state.scrollingVertically
+
+  getMousewheelScreenRow: -> @mousewheelScreenRow
+
   setBaseCharacterWidth: (@baseCharacterWidth) ->
     @characterWidthsChanged()