view transition completes before components mounted with experimental async enabled

[jeefave]

Describe the bug

View transition completes before the children are mounted leading to transitioning to the same UI before DOM is swapped. This is happening when async functions are awaited during component initilization.

Reproduction

+layout.svelte

   onNavigate((navigation) => {
        if (!document.startViewTransition || navigation.willUnload) return;

        return new Promise((resolve) => {
            document.startViewTransition(async () => {
                const s = (performance.now());
                if (navigation.to?.url) {
                    await Promise.all([preloadCode(navigation.to.url.pathname), preloadData(navigation.to.url.toString())]);
                }

                resolve();
                console.log('transition resolved', performance.now() - s);
                await navigation.complete;
                console.log('nav completed', performance.now() - s);
            });
        });
    });

+page.svelte

    await new Promise(resolve => setTimeout(resolve, 500));
    onMount(() => {
        console.log('page ready')
    });

I initially reproduced enabling network throttling while awaiting remote functions within component initlization. Note that {#await} works as intended.

Logs

transition resolved 1.399999976158142
+layout.svelte:25 nav completed 3.5
+page.svelte:42 page ready

System Info

System:
    OS: macOS 15.5
    CPU: (14) arm64 Apple M4 Pro
    Memory: 125.22 MB / 24.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.14.0 - /Users/jee/.nvm/versions/node/v22.14.0/bin/node
    Yarn: 1.22.22 - /Users/jee/.nvm/versions/node/v22.14.0/bin/yarn
    npm: 11.6.1 - /Users/jee/.nvm/versions/node/v22.14.0/bin/npm
    bun: 1.2.15 - /Users/jee/.bun/bin/bun
  Browsers:
    Chrome: 141.0.7390.77
    Firefox: 141.0.3
    Safari: 18.5
  npmPackages:
    @sveltejs/adapter-node: ^5.3.2 => 5.3.2 
    @sveltejs/kit: ^2.46.5 => 2.46.5 
    @sveltejs/vite-plugin-svelte: ^6.1.4 => 6.2.0 
    svelte: ^5.39.12 => 5.39.12 
    vite: ^7.1.9 => 7.1.9

Severity

annoyance

Additional Information

No response

[sillvva]

I added an await sleep(100) to my transition and it seemed to help a lot except for slow pages. It's a decent workaround though for the most part.