Do not reset transfer_arena if page navigation results in delayed navigation

We normally expect a navigation event to happen at some point after the page
loads, like a puppeteer script clicking on a link. But, it's also possible for
the main navigation event to result in a delayed navigation. For example, an
html page with this JS:

<script>top.location = '/';</script>

Would result in a delayed navigation being called from the main navigate
function. In these cases, we cannot clear the transfer_arena when navigate is
completed, as its memory is needed by the new "sub" delayed navigation.

src/browser/page.zig

@@ -92,6 +92,9 @@ pub const Page = struct {
     // current_script could by fetch module to resolve module's url to fetch.
     current_script: ?*const Script = null,
 
+    // indicates intention to navigate to another page on the next loop execution.
+    delayed_navigation: bool = false,
+
     pub fn init(self: *Page, arena: Allocator, session: *Session) !void {
         const browser = session.browser;
         self.* = .{
@@ -580,6 +583,7 @@ pub const Page = struct {
     // The page.arena is safe to use here, but the transfer_arena exists
     // specifically for this type of lifetime.
     pub fn navigateFromWebAPI(self: *Page, url: []const u8, opts: NavigateOpts) !void {
+        self.delayed_navigation = true;
         const arena = self.session.transfer_arena;
         const navi = try arena.create(DelayedNavigation);
         navi.* = .{

src/browser/session.zig

@@ -128,7 +128,14 @@ pub const Session = struct {
         // it isn't null!
         std.debug.assert(self.page != null);
 
-        defer _ = self.browser.transfer_arena.reset(.{ .retain_with_limit = 1 * 1024 * 1024 });
+        defer if (self.page) |*p| {
+            if (!p.delayed_navigation) {
+                // If, while loading the page, we intend to navigate to another
+                // page, then we need to keep the transfer_arena around, as this
+                // sub-navigation is probably using it.
+                _ = self.browser.transfer_arena.reset(.{ .retain_with_limit = 1 * 1024 * 1024 });
+            }
+        };
 
         // it's safe to use the transfer arena here, because the page will
         // eventually clone the URL using its own page_arena (after it gets