From a8ca5127c959e133fbba79fa8343d1569960723f Mon Sep 17 00:00:00 2001
From: Raymond Hill <rhill@raymondhill.net>
Date: Fri, 15 Jul 2022 13:35:56 -0400
Subject: [PATCH] Better detect invalid cosmetic filters

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/2170
---
 src/js/static-filtering-parser.js | 55 +++++++++++++++----------------
 1 file changed, 26 insertions(+), 29 deletions(-)

diff --git a/src/js/static-filtering-parser.js b/src/js/static-filtering-parser.js
index 9932758a3..addae1821 100644
--- a/src/js/static-filtering-parser.js
+++ b/src/js/static-filtering-parser.js
@@ -19,7 +19,7 @@
     Home: https://github.com/gorhill/uBlock
 */
 
-/* globals CSSStyleSheet, document */
+/* globals document */
 
 'use strict';
 
@@ -1333,20 +1333,17 @@ Parser.prototype.SelectorCompiler = class {
             [ 'matches-css-before', ':matches-css-before' ],
         ]);
         this.reSimpleSelector = /^[#.]?[A-Za-z_][\w-]*$/;
-        // https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet#browser_compatibility
-        //   Firefox does not support constructor for CSSStyleSheet
-        this.stylesheet = (( ) => {
+        this.styleElement = (( ) => {
             if ( typeof document !== 'object' ) { return null; }
-            if ( document instanceof Object === false ) { return null; }
+            if ( document === null ) { return null; }
             try {
-                return new CSSStyleSheet();
+                const styleElement = document.createElement('style');
+                styleElement.disabled = true;
+                document.body.append(styleElement);
+                return styleElement;
             } catch(ex) {
             }
-            const style = document.createElement('style');
-            document.body.append(style);
-            const stylesheet = style.sheet;
-            style.remove();
-            return stylesheet;
+            return null;
         })();
         this.div = (( ) => {
             if ( typeof document !== 'object' ) { return null; }
@@ -1468,18 +1465,17 @@ Parser.prototype.SelectorCompiler = class {
     //   Forbid multiple and unexpected CSS style declarations.
     sheetSelectable(s) {
         if ( this.reSimpleSelector.test(s) ) { return true; }
-        if ( this.stylesheet === null ) { return true; }
+        if ( this.styleElement === null ) { return true; }
         try {
-            this.stylesheet.insertRule(`${s}{color:red}`);
-            if ( this.stylesheet.cssRules.length !== 1 ) { return false; }
-            const style = this.stylesheet.cssRules[0].style;
+            this.styleElement.textContent = `${s}{color:red}`;
+            const rules = this.styleElement.sheet.cssRules;
+            if ( rules.length !== 1 ) { return false; }
+            const style = rules[0].style;
             if ( style.length !== 1 ) { return false; }
-            if ( style.getPropertyValue('color') !== 'red' ) { return false; }
-            this.stylesheet.deleteRule(0);
+            if ( style.getPropertyValue('color') === 'red' ) { return true; }
         } catch (ex) {
-            return false;
         }
-        return true;
+        return false;
     }
 
     // https://github.com/uBlockOrigin/uBlock-issues/issues/1806
@@ -1600,17 +1596,15 @@ Parser.prototype.SelectorCompiler = class {
     //   - opening comment `/*`
     compileStyleProperties(s) {
         if ( /image-set\(|url\(|\/\s*\/|\\|\/\*/i.test(s) ) { return; }
-        if ( this.stylesheet === null ) { return s; }
+        if ( this.styleElement === null ) { return s; }
         let valid = false;
         try {
-            this.stylesheet.insertRule(`a{${s}}`);
-            const rules = this.stylesheet.cssRules;
-            valid = rules.length !== 0 && rules[0].style.cssText !== '';
+            this.styleElement.textContent = `a{${s}} b{color:red;}`;
+            const rules = this.styleElement.sheet.cssRules;
+            valid = rules.length >= 2 &&
+                rules[0].style.cssText !== '' &&
+                rules[1].style.cssText !== '';
         } catch(ex) {
-            return;
-        }
-        if ( this.stylesheet.cssRules.length !== 0 ) {
-            this.stylesheet.deleteRule(0);
         }
         if ( valid ) { return s; }
     }
@@ -1781,8 +1775,12 @@ Parser.prototype.SelectorCompiler = class {
             if ( i === n ) { break; }
         }
 
+        // when there is an explicit action, nothing should be left to parse.
+        if ( action !== undefined && opPrefixBeg < n ) { return; }
+
         // No task found: then we have a CSS selector.
-        // At least one task found: nothing should be left to parse.
+        // At least one task found: either nothing or a valid plain CSS selector
+        // should be left to parse
         if ( tasks.length === 0 ) {
             if ( action === undefined ) {
                 prefix = raw;
@@ -1796,7 +1794,6 @@ Parser.prototype.SelectorCompiler = class {
             }
 
         } else if ( opPrefixBeg < n ) {
-            if ( action !== undefined ) { return; }
             const spath = this.compileSpathExpression(raw.slice(opPrefixBeg));
             if ( spath === undefined ) { return; }
             tasks.push([ ':spath', spath ]);