Simplify visual diff code

Remove the unnecessary per-target hasDiff flag.

Extract the code for calculating the hasDiff/showDiff flags
into a separate method.

h/static/scripts/directives/annotation.coffee

@@ -128,6 +128,18 @@ AnnotationController = [
         @action = 'view'
         @editing = false
 
+    ###*
+    # @ngdoc method
+    # @name annotation.AnnotationController.diffFromTarget
+    # @description Calculates the visual diff flags from the targets
+    ###
+    this.diffFromTargets = (targets) ->
+      hasDiff = targets.filter((t) -> t.diffHTML?).length > 0
+      shouldShowDiff = hasDiff and
+        targets.filter((t) -> t.diffHTML? and not t.diffCaseOnly).length > 0
+
+      {hasDiff, shouldShowDiff}
+
     ###*
     # @ngdoc method
     # @name annotation.AnnotationController#save
@@ -215,16 +227,15 @@ AnnotationController = [
       @annotation.tags = ({text} for text in (model.tags or []))
 
       # Calculate the visual diff flags
-      @hasDiff = false
-      for t in @annotation.target or []
-        if t.diffHTML?
-          @hasDiff = t.hasDiff = true
-          unless t.diffCaseOnly
-            @showDiff ?= true
-        else
-          t.hasDiff = false
+      diffFlags = this.diffFromTargets(@annotation.target)
+      @hasDiff = diffFlags.hasDiff
       if @hasDiff
-        @showDiff ?= false
+        # We don't want to override the showDiff value manually changed
+        # by the user, that's why we use a conditional assignment here,
+        # instead of directly setting showDiff to the calculated value
+        @showDiff ?= diffFlags.shouldShowDiff
+      else
+        @showDiff = undefined
 
     updateTimestamp = (repeat=false) =>
       @timestamp = timeHelpers.toFuzzyString model.updated

h/templates/client/annotation.html

@@ -44,14 +44,14 @@
 <section class="annotation-target"
          ng-repeat="target in vm.annotation.target track by $index">
   <blockquote class="annotation-quote"
-              ng-hide="target.hasDiff && vm.showDiff"
+              ng-hide="target.diffHTML && vm.showDiff"
               ng-bind-html="selector.exact"
               ng-repeat="selector in target.selector
                          | filter : {'type': 'TextQuoteSelector'}
                          track by $index"></blockquote>
   <blockquote class="annotation-quote"
               ng-bind-html="target.diffHTML"
-              ng-show="target.hasDiff && vm.showDiff"></blockquote>
+              ng-show="target.diffHTML && vm.showDiff"></blockquote>
 </section>
 <div class="small pull-right"
      ng-show="vm.hasDiff">

tests/js/directives/annotation-test.coffee

@@ -148,9 +148,6 @@ describe 'h.directives.annotation', ->
         assert.isTrue(controller.hasDiff)
         assert.isTrue(controller.showDiff)
 
-      it 'sets `hasDiff` on the target to true', ->
-        assert.isTrue(targets[0].hasDiff)
-
     describe 'when thre are only upper case/lower case difference between the text in the single target, and what was saved in a selector', ->
       targets = null
 
@@ -166,9 +163,6 @@ describe 'h.directives.annotation', ->
         assert.isTrue(controller.hasDiff)
         assert.isFalse(controller.showDiff)
 
-      it 'sets `hasDiff` on the target to true', ->
-        assert.isTrue(targets[0].hasDiff)
-
     describe 'when there are multiple targets, some with differences in text', ->
       targets = null
 
@@ -187,11 +181,6 @@ describe 'h.directives.annotation', ->
       it 'sets `showDiff` to true', ->
         assert.isTrue(controller.showDiff)
 
-      it 'sets `hasDiff` to true on targets with any differences', ->
-        assert.isFalse(targets[0].hasDiff)
-        assert.isTrue(targets[1].hasDiff)
-        assert.isTrue(targets[2].hasDiff)
-
       it 'preserves the `showDiff` value on update', ->
         controller.showDiff = false
         annotation.target = annotation.target.slice(1)