Add a "pre" polyfill

This is always run, but only the full webcomponents polyfill, it's very
small and isn't intrusive. This introduces a layer of indirection so that,
if the full polyfill is loaded, its monkeypatched constructor will be called

src/browser/page.zig

@@ -120,6 +120,7 @@ pub const Page = struct {
             .main_context = undefined,
         };
         self.main_context = try session.executor.createJsContext(&self.window, self, self, true, Env.GlobalMissingCallback.init(&self.polyfill_loader));
+        try polyfill.preload(self.arena, self.main_context);
 
         // message loop must run only non-test env
         if (comptime !builtin.is_test) {

src/browser/polyfill/polyfill.zig

@@ -71,7 +71,6 @@ pub const Loader = struct {
         if (!self.done.webcomponents and isWebcomponents(name)) {
             const source = @import("webcomponents.zig").source;
             self.load("webcomponents", source, js_context);
-
             // We return false here: We want v8 to continue the calling chain
             // to finally find the polyfill we just inserted. If we want to
             // return false and stops the call chain, we have to use
@@ -103,3 +102,19 @@ pub const Loader = struct {
         return false;
     }
 };
+
+pub fn preload(allocator: Allocator, js_context: *Env.JsContext) !void {
+    var try_catch: Env.TryCatch = undefined;
+    try_catch.init(js_context);
+    defer try_catch.deinit();
+
+    const name = "webcomponents-pre";
+    const source = @import("webcomponents.zig").pre;
+    _ = js_context.exec(source, name) catch |err| {
+        if (try try_catch.err(allocator)) |msg| {
+            defer allocator.free(msg);
+            log.fatal(.app, "polyfill error", .{ .name = name, .err = msg });
+        }
+        return err;
+    };
+}

src/browser/polyfill/webcomponents.zig

@@ -6,21 +6,51 @@
 // This is the `webcomponents-ce.js` bundle
 pub const source = @embedFile("webcomponents.js");
 
+// The main webcomponents.js is lazilly loaded when window.customElements is
+// called. But, if you look at the test below, you'll notice that we declare
+// our custom element (LightPanda) before we call `customElements.define`. We
+// _have_ to declare it before we can register it.
+// That causes an issue, because the LightPanda class extends HTMLElement, which
+// hasn't been monkeypatched by the polyfill yet. If you were to try it as-is
+// you'd get an "Illegal Constructor", because that's what the Zig HTMLElement
+// constructor does (and that's correct).
+// However, once HTMLElement is monkeypatched, it'll work. One simple solution
+// is to run the webcomponents.js polyfill proactively on each page, ensuring
+// that HTMLElement is monkeypatched before any other JavaScript is run. But
+// that adds _a lot_ of overhead.
+// So instead of always running the [large and intrusive] webcomponents.js
+// polyfill, we'll always run this little snippet. It wraps the HTMLElement
+// constructor. When the Lightpanda class is created, it'll extend our little
+// wrapper. But, unlike the Zig default constructor which throws, our code
+// calls the "real" constructor. That might seem like the same thing, but by the
+// time our wrapper is called, the webcomponents.js polyfill will have been
+// loaded and the "real" constructor will be the monkeypatched version.
+// TL;DR creates a layer of indirection for the constructor, so that, when it's
+// actually instantiated, the webcomponents.js polyfill will have been loaded.
+pub const pre =
+    \\ (() => {
+    \\   const HE = window.HTMLElement;
+    \\   const b = function() { return HE.prototype.constructor.call(this); }
+    \\   b.prototype = HE.prototype;
+    \\   window.HTMLElement = b;
+    \\ })();
+;
+
 const testing = @import("../../testing.zig");
 test "Browser.webcomponents" {
     var runner = try testing.jsRunner(testing.tracking_allocator, .{ .html = "<div id=main></div>" });
     defer runner.deinit();
 
+    try @import("polyfill.zig").preload(testing.allocator, runner.page.main_context);
+
     try runner.testCases(&.{
         .{
-            \\ window.customElements; // temporarily needed, lazy loading doesn't work!
-            \\
             \\ class LightPanda extends HTMLElement {
             \\   constructor() {
             \\     super();
             \\   }
             \\   connectedCallback() {
-            \\     this.append('connected')
+            \\     this.append('connected');
             \\   }
             \\ }
             \\ window.customElements.define("lightpanda-test", LightPanda);

src/cdp/domains/page.zig

@@ -284,6 +284,9 @@ pub fn pageCreated(bc: anytype, page: *Page) !void {
     if (bc.isolated_world) |*isolated_world| {
         // We need to recreate the isolated world context
         try isolated_world.createContext(page);
+
+        const polyfill = @import("../../browser/polyfill/polyfill.zig");
+        try polyfill.preload(bc.arena, &isolated_world.executor.js_context.?);
     }
 }
 

src/main_wpt.zig

@@ -126,6 +126,8 @@ fn run(
     });
     defer runner.deinit();
 
+    try polyfill.preload(arena, runner.page.main_context);
+
     // loop over the scripts.
     const doc = parser.documentHTMLToDocument(runner.page.window.document);
     const scripts = try parser.documentGetElementsByTagName(doc, "script");