Restore visual diff to original behavior

This restores the following corner case:

When there is a difference, but it's only about
the case of the characters, don't want to show
the visual diff by default, but we offer the
feature to the user.

I also updated the tests the reflect the specs.

h/static/scripts/directives/annotation.coffee

@@ -217,11 +217,16 @@ AnnotationController = [
       # Calculate the visual diff flags
       @hasDiff = false
       for t in @annotation.target or []
-        if t.diffHTML? and not t.diffCaseOnly
+        if t.diffHTML?
           @hasDiff = t.hasDiff = true
+          unless t.diffCaseOnly
+            @showDiff ?= true
         else
           t.hasDiff = false
-      @showDiff ?= @hasDiff or undefined
+      if @hasDiff
+        @showDiff ?= false
+      else
+        @showDiff ?= undefined
 
     updateTimestamp = (repeat=false) =>
       @timestamp = timeHelpers.toFuzzyString model.updated

tests/js/directives/annotation-test.coffee

@@ -151,12 +151,10 @@ describe 'h.directives.annotation', ->
       it 'sets `showDiff` to true', ->
         assert.isTrue(controller.showDiff)
 
-      it 'sets `hasDiff` to true on targets with differences', ->
+      it 'sets `hasDiff` to true on targets with any differences', ->
         assert.isFalse(targets[0].hasDiff)
         assert.isTrue(targets[1].hasDiff)
-
-      it 'sets `hasDiff` to false on targets with only case differences', ->
-        assert.isFalse(targets[2].hasDiff)
+        assert.isTrue(targets[2].hasDiff)
 
       it 'preserves the `showDiff` value on update', ->
         controller.showDiff = false
@@ -169,6 +167,34 @@ describe 'h.directives.annotation', ->
         controller.render()
         assert.isFalse(controller.hasDiff)
 
+    describe 'when targets have almost the same selection text from the anchor', ->
+      targets = null
+
+      beforeEach ->
+        annotation.target = [
+          {otherProperty: 'bar'},
+          {diffHTML: "stuff", diffCaseOnly: true},
+        ]
+        controller.render()
+        targets = controller.annotation.target
+
+      it 'sets `hasDiff` to true', ->
+        assert.isTrue(controller.hasDiff)
+
+      it 'sets `showDiff` to false', ->
+        assert.isFalse(controller.showDiff)
+
+      it 'preserves the `showDiff` value on update', ->
+        controller.showDiff = true
+        annotation.target = annotation.target.slice(1)
+        controller.render()
+        assert.isTrue(controller.showDiff)
+
+      it 'unsets `hasDiff` if differences go away', ->
+        annotation.target = annotation.target.splice(0, 1)
+        controller.render()
+        assert.isFalse(controller.hasDiff)
+
     describe 'timestamp', ->
       clock = null