Fix anchoring failure when range starts in an element node with no children

The range normalization code failed to take account of ranges where the
start container was an empty element with no children.

Fixes #73

h/static/scripts/annotator/anchoring/range.coffee

@@ -163,7 +163,10 @@ class Range.BrowserRange
     # Look at the start
     if @startContainer.nodeType is Node.ELEMENT_NODE
       # We are dealing with element nodes
-      r.start = Util.getFirstTextNodeNotBefore @startContainer.childNodes[@startOffset]
+      if @startOffset < @startContainer.childNodes.length
+        r.start = Util.getFirstTextNodeNotBefore @startContainer.childNodes[@startOffset]
+      else
+        r.start = Util.getFirstTextNodeNotBefore @startContainer
       r.startOffset = 0
     else
       # We are dealing with simple text nodes

h/static/scripts/annotator/anchoring/test/html-anchoring-fixture.html

@@ -22,3 +22,8 @@
 <p><span><!--anchor--></span><!--after-->
   Fusce fermentum sit amet augue sed rutrum.
 </p>
+
+<!-- See hypothesis/client#73 !-->
+<gh-73>
+  <div></div><span>Test</span>
+</gh-73>

h/static/scripts/annotator/anchoring/test/html-test.js

@@ -72,6 +72,8 @@ var rangeSpecs = [
   // Where the *Container nodes are expressed as either an XPath relative to
   // the container or the index into the list of text nodes within the container
   // node
+
+  // Test cases from Annotator v1.2.x's range_spec.coffee
   [ 0,           13,  0,           27,  'habitant morbi',                                    'Partial node contents.' ],
   [ 0,           0,   0,           37,  'Pellentesque habitant morbi tristique',             'Full node contents, textNode refs.' ],
   [ '/p/strong', 0,   '/p/strong', 1,   'Pellentesque habitant morbi tristique',             'Full node contents, elementNode refs.' ],
@@ -98,6 +100,9 @@ var rangeSpecs = [
   [ '/div[2]',   2,'/div[2]/text()[2]',28,'Lorem sed do eiusmod tempor.',                    'Text between br tags, with <p><br/></p> at the start'],
   [ '/div[2]',   1,'/div[2]/text()[2]',28,'Lorem sed do eiusmod tempor.',                    'Text between br tags, with <br/><p><br/></p> at the start'],
   [ '/h2[2]',    0,'/p[4]', 0, 'Header Level 2\n\n\n  Mauris lacinia ipsum nulla, id iaculis quam egestas quis.\n\n\n', 'No text node at the end and offset 0'],
+
+  // Additional test cases
+  [ '/gh-73/div', 0, '/gh-73/span/text()', 4, 'Test', 'Range starting at an element node with no children'],
 ];
 
 /**
@@ -118,6 +123,7 @@ var expectedFailures = [
   ['Text between br tags, with <p><br/></p> at the start', {position: true, quote: true}],
   ['Text between br tags, with <br/><p><br/></p> at the start', {position: true, quote: true}],
   ['No text node at the end and offset 0', {position: true, quote: true}],
+  ['Range starting at an element node with no children', {position: true, quote: true}],
 ];
 
 describe('HTML anchoring', function () {