🐎 Speed up update dur to decoration's marker changes

When a marker’s range changes we don’t want to redraw the whole
decoration surface, only the part that were added/removed.

lib/mixins/decoration-management.coffee

@@ -193,12 +193,17 @@ class DecorationManagement extends Mixin
         for decoration in decorations
           @emitter.emit 'did-change-decoration', {marker, decoration, event}
 
-      start = event.oldTailScreenPosition
-      end = event.oldHeadScreenPosition
+      oldStart = event.oldTailScreenPosition
+      oldEnd = event.oldHeadScreenPosition
 
-      [start, end] = [end, start] if start.row > end.row
+      newStart = event.newTailScreenPosition
+      newEnd = event.newHeadScreenPosition
 
-      @emitRangeChanges({start, end})
+      [oldStart, oldEnd] = [oldEnd, oldStart] if oldStart.row > oldEnd.row
+      [newStart, newEnd] = [newEnd, newStart] if newStart.row > newEnd.row
+
+      rangesDiffs = @computeRangesDiffs(oldStart, oldEnd, newStart, newEnd)
+      @emitRangeChanges({start, end}, 0) for [start, end] in rangesDiffs
 
     decoration = new Decoration(marker, this, decorationParams)
     @decorationsByMarkerId[marker.id] ?= []
@@ -215,6 +220,21 @@ class DecorationManagement extends Mixin
     @emitter.emit 'did-add-decoration', {marker, decoration}
     decoration
 
+  computeRangesDiffs: (oldStart, oldEnd, newStart, newEnd) ->
+    diffs = []
+
+    if oldStart.isLessThan(newStart)
+      diffs.push([oldStart, newStart])
+    else if newStart.isLessThan(oldStart)
+      diffs.push([newStart, oldStart])
+
+    if oldEnd.isLessThan(newEnd)
+      diffs.push([oldEnd, newEnd])
+    else if newEnd.isLessThan(oldEnd)
+      diffs.push([newEnd, oldEnd])
+
+    diffs
+
   # Internal: Emits a change in the {Minimap} corresponding to the
   # passed-in decoration.
   #

spec/minimap-spec.coffee

@@ -220,10 +220,12 @@ describe 'Minimap', ->
     [marker, decoration, changeSpy] = []
 
     beforeEach ->
+      editor.setText(largeSample)
+
       changeSpy = jasmine.createSpy('didChange')
       minimap.onDidChange(changeSpy)
 
-      marker = minimap.markBufferRange [[0,6], [0,11]]
+      marker = minimap.markBufferRange [[0,6], [1,11]]
       decoration = minimap.decorateMarker marker, type: 'highlight', class: 'dummy'
 
     it 'creates a decoration for the given marker', ->
@@ -232,7 +234,20 @@ describe 'Minimap', ->
     it 'creates a change corresponding to the marker range', ->
       expect(changeSpy).toHaveBeenCalled()
       expect(changeSpy.calls[0].args[0].start).toEqual(0)
-      expect(changeSpy.calls[0].args[0].end).toEqual(0)
+      expect(changeSpy.calls[0].args[0].end).toEqual(1)
+
+    describe 'when the marker range changes', ->
+      beforeEach ->
+        markerChangeSpy = jasmine.createSpy('marker-did-change')
+        marker.onDidChange(markerChangeSpy)
+        marker.setBufferRange [[0,6], [3,11]]
+
+        waitsFor -> markerChangeSpy.calls.length > 0
+
+      it 'creates a change only for the dif between the two ranges', ->
+        expect(changeSpy).toHaveBeenCalled()
+        expect(changeSpy.calls[1].args[0].start).toEqual(1)
+        expect(changeSpy.calls[1].args[0].end).toEqual(3)
 
     describe 'destroying the marker', ->
       beforeEach ->
@@ -243,7 +258,7 @@ describe 'Minimap', ->
 
       it 'creates a change corresponding to the marker range', ->
         expect(changeSpy.calls[1].args[0].start).toEqual(0)
-        expect(changeSpy.calls[1].args[0].end).toEqual(0)
+        expect(changeSpy.calls[1].args[0].end).toEqual(1)
 
     describe 'destroying the decoration', ->
       beforeEach ->
@@ -254,7 +269,7 @@ describe 'Minimap', ->
 
       it 'creates a change corresponding to the marker range', ->
         expect(changeSpy.calls[1].args[0].start).toEqual(0)
-        expect(changeSpy.calls[1].args[0].end).toEqual(0)
+        expect(changeSpy.calls[1].args[0].end).toEqual(1)
 
     describe 'destroying all the decorations for the marker', ->
       beforeEach ->
@@ -265,7 +280,7 @@ describe 'Minimap', ->
 
       it 'creates a change corresponding to the marker range', ->
         expect(changeSpy.calls[1].args[0].start).toEqual(0)
-        expect(changeSpy.calls[1].args[0].end).toEqual(0)
+        expect(changeSpy.calls[1].args[0].end).toEqual(1)
 
     describe 'destroying the minimap', ->
       beforeEach ->