Bug 1415940 Part 5: Change InspectorUtils::GetRelativeRuleLine to not remap line numbers if it introduces underflow. r=bz

MozReview-Commit-ID: 8ZhzPWubBg7

--HG--
extra : rebase_source : e3edb4a9f9449dc6c760a71cd61a3ffb08c9fcfb

devtools/client/inspector/rules/test/doc_cssom.html

@@ -9,6 +9,9 @@
     window.onload = function () {
       let x = document.styleSheets[0];
       x.insertRule("div { color: seagreen; }", 1);
+
+      // Add a rule with a leading newline, to test that inspector can handle it.
+      x.insertRule("\n\ndiv { font-weight: bold; }", 1);
     };
   </script>
 

layout/inspector/InspectorUtils.cpp

@@ -275,6 +275,28 @@ InspectorUtils::GetRuleColumn(GlobalObject& aGlobal, css::Rule& aRule)
 InspectorUtils::GetRelativeRuleLine(GlobalObject& aGlobal, css::Rule& aRule)
 {
   uint32_t lineNumber = aRule.GetLineNumber();
+
+  // If aRule was parsed along with its stylesheet, then it will
+  // have an absolute lineNumber that we need to remap to its
+  // containing node. But if aRule was added via CSSOM after parsing,
+  // then it has a sort-of relative line number already:
+  // Gecko gives all rules a 0 lineNumber.
+  // Servo gives the first line of a rule a 0 lineNumber, and then
+  //   counts up from there.
+
+  // The Servo behavior is arguably more correct, but harder to
+  // interpret for purposes of deciding whether a lineNumber is
+  // relative or absolute.
+
+  // Since most of the time, inserted rules are single line and
+  // therefore have 0 lineNumbers in both Gecko and Servo, we use
+  // that to detect that a lineNumber is already relative.
+
+  // There is one ugly edge case that we avoid: if an inserted rule
+  // is multi-line, then Servo will give it 0+ lineNumbers. If we
+  // do relative number mapping on those line numbers, we could get
+  // negative underflow. So we check for underflow and instead report
+  // a 0 lineNumber.
   StyleSheet* sheet = aRule.GetStyleSheet();
   if (sheet && lineNumber != 0) {
     nsINode* owningNode = sheet->GetOwnerNode();
@@ -282,7 +304,14 @@ InspectorUtils::GetRelativeRuleLine(GlobalObject& aGlobal, css::Rule& aRule)
       nsCOMPtr<nsIStyleSheetLinkingElement> link =
         do_QueryInterface(owningNode);
       if (link) {
-        lineNumber -= link->GetLineNumber() - 1;
+        // Check for underflow, which is one indication that we're
+        // trying to remap an already relative lineNumber.
+        uint32_t linkLineIndex0 = link->GetLineNumber() - 1;
+        if (linkLineIndex0 > lineNumber ) {
+          lineNumber = 0;
+        } else {
+          lineNumber -= linkLineIndex0;
+        }
       }
     }
   }