Describe the bug

I have run into a scenario where async code of any kind seems to not immediately be run/flushed. In other words $effect.pending will be stuck > 0 and things like async deriveds will not re-run even when the value they depend on changes. They will eventually be run if something else triggers a flush of pending promises.

I am not certain I have found the minimal reproduction, but after many hours I have narrowed it down to swapping out recursive dynamic components.

Reproduction

Reproduction: https://svelte.dev/playground/eee80b71655641e3b7e0eb5afcb93469?version=5.39.5

This reproduction shows a basic tree structure rendered by the Node component. The displayed Message component contains async work.

Click the "Swap children" button which will replace the existing Message with a pair of Messages nested one level deeper.

Observe that the $effect.pending() displayed is stuck > 0. Observe that the async derived that doubles the input number does not re-run after the initial calculation.

Logs

System Info

System: OS: Linux 6.15 Arch Linux CPU: (12) x64 AMD Ryzen 5 2600 Six-Core Processor Memory: 4.58 GB / 15.54 GB Container: Yes Shell: 5.3.3 - /bin/bash Binaries: Node: 18.16.0 - ~/.nvm/versions/node/v18.16.0/bin/node Yarn: 1.22.22 - /usr/bin/yarn npm: 9.5.1 - ~/.nvm/versions/node/v18.16.0/bin/npm pnpm: 10.13.1 - ~/.local/share/pnpm/pnpm Browsers: Chromium: 139.0.7258.66

Severity

blocking an upgrade