fix: avoid duplicating escaped characters when parsing styles (#17554)

* fix and test

* changeset

* format

* increment index and escape early instead

.changeset/true-berries-behave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid duplicating escaped characters in CSS AST

packages/svelte/src/compiler/phases/1-parse/read/style.js

@@ -508,8 +508,12 @@ function read_value(parser) {
 		if (escaped) {
 			value += '\\' + char;
 			escaped = false;
+			parser.index++;
+			continue;
 		} else if (char === '\\') {
 			escaped = true;
+			parser.index++;
+			continue;
 		} else if (char === quote_mark) {
 			quote_mark = null;
 		} else if (char === ')') {

packages/svelte/tests/css-parse.test.ts

@@ -135,4 +135,19 @@ describe('parseCss', () => {
 			}
 		}
 	});
+
+	it('parses escaped characters', () => {
+		const ast = parseCss("div { background: url('./example.png?\\''); }");
+		assert.equal(ast.type, 'StyleSheet');
+		assert.equal(ast.children.length, 1);
+		const rule = ast.children[0];
+		assert.equal(rule.type, 'Rule');
+		if (rule.type === 'Rule') {
+			const declaration = rule.block.children[0];
+			assert.equal(declaration.type, 'Declaration');
+			if (declaration.type === 'Declaration') {
+				assert.equal(declaration.value, "url('./example.png?\\'')");
+			}
+		}
+	});
 });