Suppress merging selections during undo/redo

Now, during undo/redo overlapping selections will be temporarily
created as markers are created via snapshots. Old selections will
immediately be destroyed though, since undo/redo now completely
replace all historied markers w/ those in the snapshot, so there
is no need to merge selections.

spec/text-editor-spec.coffee

@@ -3417,6 +3417,63 @@ describe "TextEditor", ->
         expect(buffer.lineForRow(0)).not.toContain "foo"
         expect(buffer.lineForRow(0)).toContain "fovar"
 
+      it "restores cursors and selections to their states before and after undone and redone changes", ->
+        editor.setSelectedBufferRanges([
+          [[0, 0], [0, 0]],
+          [[1, 0], [1, 3]],
+        ])
+        editor.insertText("abc")
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[0, 3], [0, 3]],
+          [[1, 3], [1, 3]]
+        ]
+
+        editor.setCursorBufferPosition([0, 0])
+        editor.setSelectedBufferRanges([
+          [[2, 0], [2, 0]],
+          [[3, 0], [3, 0]],
+          [[4, 0], [4, 3]],
+        ])
+        editor.insertText("def")
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[2, 3], [2, 3]],
+          [[3, 3], [3, 3]]
+          [[4, 3], [4, 3]]
+        ]
+
+        editor.setCursorBufferPosition([0, 0])
+        editor.undo()
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[2, 0], [2, 0]],
+          [[3, 0], [3, 0]],
+          [[4, 0], [4, 3]],
+        ]
+
+        editor.undo()
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[0, 0], [0, 0]],
+          [[1, 0], [1, 3]]
+        ]
+
+        editor.redo()
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[0, 3], [0, 3]],
+          [[1, 3], [1, 3]]
+        ]
+
+        editor.redo()
+
+        expect(editor.getSelectedBufferRanges()).toEqual [
+          [[2, 3], [2, 3]],
+          [[3, 3], [3, 3]]
+          [[4, 3], [4, 3]]
+        ]
+
       it "restores the selected ranges after undo and redo", ->
         editor.setSelectedBufferRanges([[[1, 6], [1, 10]], [[1, 22], [1, 27]]])
         editor.delete()

src/text-editor.coffee

@@ -1119,12 +1119,12 @@ class TextEditor extends Model
 
   # Essential: Undo the last change.
   undo: ->
-    @buffer.undo()
+    @avoidMergingSelections => @buffer.undo()
     @getLastSelection().autoscroll()
 
   # Essential: Redo the last change.
   redo: ->
-    @buffer.redo(this)
+    @avoidMergingSelections => @buffer.redo()
     @getLastSelection().autoscroll()
 
   # Extended: Batch multiple operations as a single undo/redo step.
@@ -2217,6 +2217,9 @@ class TextEditor extends Model
 
       previousSelection.intersectsScreenRowRange(screenRange.start.row, screenRange.end.row)
 
+  avoidMergingSelections: (args...) ->
+    @mergeSelections args..., -> false
+
   mergeSelections: (args...) ->
     mergePredicate = args.pop()
     fn = args.pop() if _.isFunction(_.last(args))