Scroll React editor all the way left when soft wrap is enabled

Fixes #2842

spec/display-buffer-spec.coffee

@@ -215,6 +215,19 @@ describe "DisplayBuffer", ->
         displayBuffer.setEditorWidthInChars(-1)
         expect(displayBuffer.editorWidthInChars).not.toBe -1
 
+    it "sets ::scrollLeft to 0 and keeps it there when soft wrapping is enabled", ->
+      displayBuffer.setDefaultCharWidth(10)
+      displayBuffer.setWidth(50)
+      displayBuffer.manageScrollPosition = true
+
+      displayBuffer.setSoftWrap(false)
+      displayBuffer.setScrollLeft(Infinity)
+      expect(displayBuffer.getScrollLeft()).toBeGreaterThan 0
+      displayBuffer.setSoftWrap(true)
+      expect(displayBuffer.getScrollLeft()).toBe 0
+      displayBuffer.setScrollLeft(10)
+      expect(displayBuffer.getScrollLeft()).toBe 0
+
   describe "primitive folding", ->
     beforeEach ->
       displayBuffer.destroy()

src/display-buffer.coffee

@@ -204,6 +204,9 @@ class DisplayBuffer extends Model
     else
       @scrollLeft = Math.round(scrollLeft)
 
+  getMaxScrollLeft: ->
+    @getScrollWidth() - @getClientWidth()
+
   getScrollRight: -> @scrollLeft + @width
   setScrollRight: (scrollRight) ->
     @setScrollLeft(scrollRight - @width)
@@ -1075,7 +1078,9 @@ class DisplayBuffer extends Model
     @computeScrollWidth() if oldMaxLineLength isnt @maxLineLength
 
   computeScrollWidth: ->
-    @scrollWidth = @pixelPositionForScreenPosition([@longestScreenRow, @maxLineLength]).left + 1
+    @scrollWidth = @pixelPositionForScreenPosition([@longestScreenRow, @maxLineLength]).left
+    @scrollWidth += 1 unless @getSoftWrap()
+    @setScrollLeft(Math.min(@getScrollLeft(), @getMaxScrollLeft()))
 
   handleBufferMarkersUpdated: =>
     if event = @pendingChangeEvent