preserve symmetry when walking css ast

Author: Rich-Harris

src/compiler/compile/css/Stylesheet.ts

@@ -269,24 +269,19 @@ export default class Stylesheet {
 
 			this.has_styles = true;
 
-			const stack: Array<Rule | Atrule> = [];
+			const stack: Array<Atrule> = [];
+			let depth = 0;
 			let current_atrule: Atrule = null;
 
 			walk(ast.css, {
 				enter: (node: Node) => {
 					if (node.type === 'Atrule') {
-						const last = stack[stack.length - 1];
-
 						const atrule = new Atrule(node);
 						stack.push(atrule);
 
-						// this is an awkward special case — @apply (and
-						// possibly other future constructs)
-						if (last && !(last instanceof Atrule)) return;
-
 						if (current_atrule) {
 							current_atrule.children.push(atrule);
-						} else {
+						} else if (depth <= 1) {
 							this.children.push(atrule);
 						}
 
@@ -303,26 +298,24 @@ export default class Stylesheet {
 
 					if (node.type === 'Rule') {
 						const rule = new Rule(node, this, current_atrule);
-						stack.push(rule);
 
 						if (current_atrule) {
 							current_atrule.children.push(rule);
-						} else {
+						} else if (depth <= 1) {
 							this.children.push(rule);
 						}
 					}
+
+					depth += 1;
 				},
 
 				leave: (node: Node) => {
-					if (node.type === 'Rule' || node.type === 'Atrule') stack.pop();
 					if (node.type === 'Atrule') {
-						current_atrule = null;
-						for (let i = stack.length - 1; i >= 0; i--) {
-							if (stack[i] instanceof Atrule) {
-								current_atrule = stack[i] as Atrule;
-							}
-						}
+						stack.pop();
+						current_atrule = stack[stack.length - 1];
 					}
+
+					depth -= 1;
 				}
 			});
 		} else {