Handle selection dragging when cursor goes off-screen

spec/app/editor-spec.coffee

@@ -712,7 +712,7 @@ describe "Editor", ->
         editor.renderedLines.trigger mousedownEvent(editor: editor, point: [4, 10])
 
         # moving changes selection
-        editor.renderedLines.trigger mousemoveEvent(editor: editor, point: [5, 27])
+        $(document).trigger mousemoveEvent(editor: editor, point: [5, 27])
 
         range = editor.getSelection().getScreenRange()
         expect(range.start).toEqual({row: 4, column: 10})
@@ -730,6 +730,24 @@ describe "Editor", ->
         expect(range.end).toEqual({row: 5, column: 27})
         expect(editor.getCursorScreenPosition()).toEqual(row: 5, column: 27)
 
+      it "selects and scrolls if the mouse is dragged outside of the editor itself", ->
+        intervalFns = []
+        editor.attachToDom(heightInLines: 5)
+        editor.scrollToBottom()
+
+        spyOn(window, 'setInterval').andCallFake (fn) -> intervalFns.push(fn)
+        # start
+        editor.renderedLines.trigger mousedownEvent(editor: editor, point: [12, 0])
+
+        # moving changes selection
+        $(document).trigger mousemoveEvent(editor: editor, pageX: 0, pageY: -10)
+        expect(editor.scrollTop()).toBe 4 * editor.lineHeight
+
+        # if cursor stays off screen, we keep moving / scrolling up
+        fn() for fn in intervalFns
+
+        expect(editor.scrollTop()).toBe 0
+
     describe "double-click and drag", ->
       it "selects the word under the cursor, then continues to select by word in either direction as the mouse is dragged", ->
         expect(editor.getCursorScreenPosition()).toEqual(row: 0, column: 0)

src/app/editor.coffee

@@ -358,10 +358,18 @@ class Editor extends View
         @gutter.addClass('drop-shadow')
 
   selectOnMousemoveUntilMouseup: ->
-    moveHandler = (e) => @selectToScreenPosition(@screenPositionFromMouseEvent(e))
-    @on 'mousemove', moveHandler
+    lastMoveEvent = null
+    moveHandler = (event = lastMoveEvent) =>
+      if event
+        @selectToScreenPosition(@screenPositionFromMouseEvent(event))
+        lastMoveEvent = event
+
+    $(document).on 'mousemove', moveHandler
+    interval = setInterval(moveHandler, 20)
+
     $(document).one 'mouseup', =>
-      @off 'mousemove', moveHandler
+      clearInterval(interval)
+      $(document).off 'mousemove', moveHandler
       reverse = @activeEditSession.getLastSelection().isReversed()
       @activeEditSession.mergeIntersectingSelections({reverse})
       @activeEditSession.finalizeSelections()