Pass old/new buffer and screen positions to marker position observers

This is needed by the snippets package, which needs to know where the
cursor was previously when it moves to decide whether to cancel the
current snippet.

src/app/buffer-marker.coffee

@@ -36,9 +36,12 @@ class BufferMarker
   getTailPosition: -> @tailPosition
 
   setHeadPosition: (headPosition, options={}) ->
+    oldPosition = @headPosition
     @headPosition = Point.fromObject(headPosition)
     @headPosition = @buffer.clipPosition(@headPosition) if options.clip ? true
-    observer(@headPosition, !!options.bufferChanged) for observer in @headPositionObservers
+    newPosition = @headPosition
+    bufferChanged = !!options.bufferChanged
+    observer({oldPosition, newPosition, bufferChanged}) for observer in @headPositionObservers
     @headPosition
 
   setTailPosition: (tailPosition, options={}) ->

src/app/cursor.coffee

@@ -12,10 +12,10 @@ class Cursor
   needsAutoscroll: null
 
   constructor: ({@editSession, @marker}) ->
-    @editSession.observeMarkerHeadScreenPosition @marker, (screenPosition, bufferChanged) =>
-      @needsAutoscroll ?= @isLastCursor() and !bufferChanged
-      @trigger 'moved', screenPosition
-      @editSession.trigger 'cursor-moved', screenPosition
+    @editSession.observeMarkerHeadScreenPosition @marker, (e) =>
+      @needsAutoscroll ?= @isLastCursor() and !e.bufferChanged
+      @trigger 'moved', e
+      @editSession.trigger 'cursor-moved', e
     @needsAutoscroll = true
 
   destroy: ->

src/app/display-buffer.coffee

@@ -372,9 +372,14 @@ class DisplayBuffer
     @markerScreenPositionObservers[id] ?= { head: [], tail: [] }
     @cacheMarkerScreenPositions(id) unless @markerScreenPositions[id]
     @markerScreenPositionObservers[id].head.push(callback)
-    subscription = @buffer.observeMarkerHeadPosition id, (bufferPosition, bufferChanged) =>
+    subscription = @buffer.observeMarkerHeadPosition id, (e) =>
+      bufferChanged = e.bufferChanged
+      oldBufferPosition = e.oldPosition
+      newBufferPosition = e.newPosition
+      oldScreenPosition = @markerScreenPositions[id].head
       @cacheMarkerScreenPositions(id)
-      callback(@getMarkerHeadScreenPosition(id), bufferChanged)
+      newScreenPosition = @getMarkerHeadScreenPosition(id)
+      callback({ oldBufferPosition, newBufferPosition, oldScreenPosition, newScreenPosition, bufferChanged })
 
     cancel: =>
       subscription.cancel()
@@ -391,8 +396,13 @@ class DisplayBuffer
     for id, { head } of @markerScreenPositions
       currentHeadPosition = @getMarkerHeadScreenPosition(id)
       unless currentHeadPosition.isEqual(head)
+        bufferChanged = false
+        oldBufferPosition = newBufferPosition = @buffer.getMarkerHeadPosition(id)
+        oldScreenPosition = @markerScreenPositions[id].head
         @cacheMarkerScreenPositions(id)
-        observer(currentHeadPosition, false) for observer in @markerScreenPositionObservers[id].head
+        newScreenPosition = @getMarkerHeadScreenPosition(id)
+        for observer in @markerScreenPositionObservers[id].head
+          observer({oldScreenPosition, newScreenPosition, oldBufferPosition, newBufferPosition, bufferChanged})
 
   destroy: ->
     @tokenizedBuffer.destroy()