Don't destroy selection/cursor anchors when encompassed by a change

Add the 'strong' option to anchors. If anchors are 'strong' instead of being destroyed by encompassing changes they move to the beginning of the change range.

spec/app/edit-session-spec.coffee

@@ -1276,6 +1276,19 @@ describe "EditSession", ->
         expect(cursor2.getScreenPosition()).toEqual [0, 8]
         expect(cursor3.getScreenPosition()).toEqual [1, 0]
 
+      it "does not destroy cursor or selection anchors when a change encompasses them", ->
+        cursor = editSession.getLastCursor()
+        cursor.setBufferPosition [3, 3]
+        editSession.buffer.delete([[3, 1], [3, 5]])
+        expect(cursor.getBufferPosition()).toEqual [3, 1]
+        expect(editSession.getAnchors().indexOf(cursor.anchor)).not.toBe -1
+
+        selection = editSession.getLastSelection()
+        selection.setBufferRange [[3, 5], [3, 10]]
+        editSession.buffer.delete [[3, 3], [3, 8]]
+        expect(selection.getBufferRange()).toEqual [[3, 3], [3, 5]]
+        expect(editSession.getAnchors().indexOf(selection.anchor)).not.toBe -1
+
       it "merges cursors when the change causes them to overlap", ->
         editSession.setCursorScreenPosition([0, 0])
         editSession.addCursorAtScreenPosition([0, 1])

src/app/anchor.coffee

@@ -9,14 +9,17 @@ class Anchor
   screenPosition: null
 
   constructor: (@editSession, options = {}) ->
-    { @ignoreEqual } = options
+    { @ignoreEqual, @strong } = options
 
   handleBufferChange: (e) ->
     { oldRange, newRange } = e
     position = @getBufferPosition()
 
     if oldRange.containsPoint(position, exclusive: true)
-      @destroy()
+      if @strong
+        @setBufferPosition(oldRange.start)
+      else
+        @destroy()
       return
 
     if @ignoreEqual

src/app/cursor.coffee

@@ -12,7 +12,7 @@ class Cursor
   wordRegex: /(\w+)|([^\w\s]+)/g
 
   constructor: ({@editSession, screenPosition, bufferPosition}) ->
-    @anchor = @editSession.addAnchor()
+    @anchor = @editSession.addAnchor(strong: true)
     @anchor.on 'change-screen-position', (args...) => @trigger 'change-screen-position', args...
     @setScreenPosition(screenPosition) if screenPosition
     @setBufferPosition(bufferPosition) if bufferPosition

src/app/selection.coffee

@@ -226,7 +226,7 @@ class Selection
       @trigger 'change-screen-range', newScreenRange unless oldScreenRange.isEqual(newScreenRange)
 
   placeAnchor: ->
-    @anchor = @editSession.addAnchor()
+    @anchor = @editSession.addAnchor(strong: true)
     @anchor.setScreenPosition(@cursor.getScreenPosition())
     @anchor.on 'change-screen-position.selection', => @trigger 'change-screen-range'