Only preserve mouseWheelScreenRow if it's out of the rendered row range

Fixes #2429, #2443

Otherwise, it's possible to duplicate lines. If a line is in the
rendered row range and it's not in the set of lines returned by the
editor, we should remove it no matter what. Line preservation is only
intended for lines that are out of view.

spec/editor-component-spec.coffee

@@ -788,6 +788,21 @@ describe "EditorComponent", ->
 
           expect(component.mouseWheelScreenRow).toBe null
 
+        it "does not preserve the line if it is on screen", ->
+          expect(node.querySelectorAll('.line-number').length).toBe 14 # dummy line
+          lineNodes = node.querySelectorAll('.line')
+          expect(lineNodes.length).toBe 13
+          lineNode = lineNodes[0]
+
+          wheelEvent = new WheelEvent('mousewheel', wheelDeltaX: 0, wheelDeltaY: 100) # goes nowhere, we're already at scrollTop 0
+          Object.defineProperty(wheelEvent, 'target', get: -> lineNode)
+          node.dispatchEvent(wheelEvent)
+
+          expect(component.mouseWheelScreenRow).toBe 0
+          editor.insertText("hello")
+          expect(node.querySelectorAll('.line-number').length).toBe 14 # dummy line
+          expect(node.querySelectorAll('.line').length).toBe 13
+
       describe "when the mousewheel event's target is a line number", ->
         it "keeps the line number on the DOM if it is scrolled off-screen", ->
           node.style.height = 4.5 * lineHeightInPixels + 'px'

src/editor-component.coffee

@@ -38,6 +38,7 @@ EditorComponent = React.createClass
 
     if @isMounted()
       renderedRowRange = @getRenderedRowRange()
+      [renderedStartRow, renderedEndRow] = renderedRowRange
       scrollHeight = editor.getScrollHeight()
       scrollWidth = editor.getScrollWidth()
       scrollTop = editor.getScrollTop()
@@ -48,6 +49,8 @@ EditorComponent = React.createClass
       verticalScrollbarWidth = editor.getVerticalScrollbarWidth()
       verticallyScrollable = editor.verticallyScrollable()
       horizontallyScrollable = editor.horizontallyScrollable()
+      if @mouseWheelScreenRow? and not (renderedStartRow <= @mouseWheelScreenRow < renderedEndRow)
+        mouseWheelScreenRow = @mouseWheelScreenRow
 
     className = 'editor editor-colors react'
     className += ' is-focused' if focused
@@ -56,7 +59,7 @@ EditorComponent = React.createClass
       GutterComponent {
         ref: 'gutter', editor, renderedRowRange, maxLineNumberDigits,
         scrollTop, scrollHeight, lineHeight, lineHeightInPixels, fontSize, fontFamily,
-        @pendingChanges, onWidthChanged: @onGutterWidthChanged, @mouseWheelScreenRow
+        @pendingChanges, onWidthChanged: @onGutterWidthChanged, mouseWheelScreenRow
       }
 
       EditorScrollViewComponent {
@@ -64,7 +67,7 @@ EditorComponent = React.createClass
         lineHeight, lineHeightInPixels, renderedRowRange, @pendingChanges,
         scrollTop, scrollLeft, scrollHeight, scrollWidth, @scrollingVertically,
         @cursorsMoved, @selectionChanged, @selectionAdded, cursorBlinkPeriod,
-        cursorBlinkResumeDelay, @onInputFocused, @onInputBlurred, @mouseWheelScreenRow,
+        cursorBlinkResumeDelay, @onInputFocused, @onInputBlurred, mouseWheelScreenRow,
         invisibles, visible, scrollViewHeight, focused
       }