Bug 1470035: Don't reparse cssom sheets, and warn about the reparse being lossy. r=jryans

MozReview-Commit-ID: 7j7Tyej0oYn

--HG--
extra : rebase_source : dd5966a2a9779175f88ae7448aa337155fe65dcd

devtools/server/actors/targets/browsing-context.js

@@ -35,6 +35,10 @@ const InspectorUtils = require("InspectorUtils");
 
 const EXTENSION_CONTENT_JSM = "resource://gre/modules/ExtensionContent.jsm";
 
+const { LocalizationHelper } = require("devtools/shared/l10n");
+const STRINGS_URI = "devtools/shared/locales/browsing-context.properties";
+const L10N = new LocalizationHelper(STRINGS_URI);
+
 const { ActorClassWithSpec, Actor } = require("devtools/shared/protocol");
 const { browsingContextTargetSpec } = require("devtools/shared/specs/targets/browsing-context");
 
@@ -1055,11 +1059,24 @@ const browsingContextTargetPrototype = {
       // Shadow DOM / XBL.
       const sheets =
         InspectorUtils.getAllStyleSheets(docShell.document, /* documentOnly = */ true);
+      const promises = [];
       for (const sheet of sheets) {
-        getSheetText(sheet, this._consoleActor).then(text => {
-          InspectorUtils.parseStyleSheet(sheet, text, /* aUpdate = */ false);
-        });
+        if (InspectorUtils.hasRulesModifiedByCSSOM(sheet)) {
+          continue;
         }
+        // Reparse the sheet so that we see the existing errors.
+        promises.push(getSheetText(sheet, this._consoleActor).then(text => {
+          InspectorUtils.parseStyleSheet(sheet, text, /* aUpdate = */ false);
+        }));
+      }
+
+      Promise.all(promises).then(() => {
+        this.logInPage({
+          text: L10N.getStr("cssSheetsReparsedWarning"),
+          category: "CSS Parser",
+          flags: Ci.nsIScriptError.warningFlag,
+        });
+      });
     }
 
     return {};

devtools/shared/locales/en-US/browsing-context.properties

@@ -0,0 +1,7 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# LOCALIZATION NOTE (cssSheetsReparsedWarning): The warning
+# to show after the CSS message filter has been toggled on.
+cssSheetsReparsedWarning=Stylesheets without CSSOM changes reparsed to check for errors. Refresh the page to also see errors from stylesheets changed from CSSOM and from style attributes.