`flushSync()` errors when called at top level of script; doesn't work with `$effect.pre`

Describe the bug

In versions 5.53.12 and later. Works in 5.53.11.

Gives error:

null is not an object (evaluating 'batch.transfer_effects') in <unknown>

Happens in async and non-async modes.

May be related to https://github.com/sveltejs/svelte/issues/18209

Reproduction

Simple reproduction in playground:

https://svelte.dev/playground/hello-world?version=5.53.12#H4sIAAAAAAAAE22PsQ6CQAyGX6V2gsRIXJGYuLk7eg4IJVxy9C53RSSEd_dAo4tb--X7_6YTctkR5ngmYywM1psaEqq1UJ3iFhttKGB-nVBGt3gLiPyTOjm3Cw8ysrB7GegfrywLscQaLELltZOjYiWKAXTnrBeYoDF9aC8jVzBD420HCt95hYcoR_1rJOlKiuzXxUW7P64fbIosjorjWaGnYC6-p_kWt1KbQXONeVOaQPMLi6-2o_kAAAA

Seems to be fixed if you add at least one $effect block:

https://svelte.dev/playground/hello-world?version=5.53.12#H4sIAAAAAAAAE22Puw7CMAxFf8VYDK2EQKylVGJjZyQMpXVEpNSJEpeHqv47KSBY2Oyjc_0YkOuOsMA9Wevg5oJtIaPWCLU5LlAbSxGL44Dy8JM3gcQ_qZ33y3glKxM715H-8caxEEsag2VsgvFSKVaiGMB03gWBAbTt4-Xw4AZG0MF1oPCdV7hJctK_RpZ_yJy0pkayLIdtBcP45uXqt4PLy7p6fTYrV6lUnM4RugsWEnoaT6mrjb0ZbrHQtY00PgFKuK8_EQEAAA

But not if you use an $effect.pre:

https://svelte.dev/playground/hello-world?version=5.53.12#H4sIAAAAAAAAE22Puw7CMAxFf8VYDK2EQKylVGJjZyQMpXVEpNSJEpeHqv47KSBY2Oyjc_0YkOuOsMA9Wevg5oJtIaPWCLU5LlAbSxGL44Dy8JM3gcQ_qZ33y3glKxM715H-8caxEEsag2VsgvFSKVaiGMB03gWBAbTt4-Xw4AZG0MF1oPCdV7hJctK_RpZ_yJy0pkayLIdtBcP45uXqt4PLy7p6fTYrV6lUnM4RugsWEnoaT6mrjb0ZbrHQtY00PgFKuK8_EQEAAA

And calling flushSync() if you have an $effect.pre block doesn’t actually run the effect, and then breaks future reactivity (also in 5.53.11):

https://svelte.dev/playground/hello-world?version=5.53.11#H4sIAAAAAAAAE21RTU_DMAz9K8baoRNoE9fSFiEu3DkSDl3qsIgsqRqXMVX97zjNBprEIYr9_D6sZELfHghLfCHnAhzD4DooqLNM3Rrv0FhHEcu3CfnUJ14CBD-rnvp-E7_IccJ2baT_cB08k2exwSrqwfbcKK9YeXvow8AwgXFj3L-evIYZzBAOoDCrFT4IVciOGCLUsIrcMhUKW4Xr82xFxpDmTT9QUayhbmBa_FmCY3C0ceFDFJlVijfcQlzEPF88krfCXc7j332Ky9yMXrMNHnYjc_DPzupP6iTt36gr0nlRziFy6hSlZY1HuWUMZe7zRimv2v69k6_2983yOzfVVkoBsj0Er1NCPV3Fzc1SnKpthkUgf8D0zVjyMNL8Ll1r3dH6DkvTukjzDxqYqxYGAgAA

Interestingly, flushSync works if it’s not called from the top level of the script, but instead e.g. with a button handler:

https://svelte.dev/playground/hello-world?version=5.56.4#H4sIAAAAAAAAE3VSXU-DMBT9K9dmDxANqI8MZtii8V0TH8QHhItr1rWE3jkXwn-35XMkWwihPT33nMPtrZlM98gC9opCKDiqSuTgYM4Jc5fdsYIL1Cz4rBmdSsuzgMH7qrgsPf2Lgiz2nWq8hGdKEkoyMizUWcVLWiUyIfvyfakqghoKcdDbt5PMoIGiUntIWFefsOVAFkhgtSolIIKFppTQSVicMHc5nOvp5L5FDb7AosCMvLJCx3EhWhk_o6OVQE-oHyPx_PLyvHnvdaybeazS6BZF0BrBEzxAAI_LgTXXAdCBIcIt6C5S447pi4PMiCsJ3wciJTeCZzvMTZ568PT9NhaBxONm_M9LGRK2Nt-g2y9n1VOYmY8Hb0jE5c-oR6pLOpm5M6Xefjo-641hfKhqpy93oecHsB5upuf0km36ER9v3jnv_hXF-JpifF2RGisb-tPoyXD7sGoH_ib0zdIAXbNAycz2K6pnzWtW7eIU-h1sCsxYE_4RC6g6YPNldikXRy5zFhSp0Nj8A4zhabtZAwAA

Or if you wrap the flushSync() call in queueMicrotask:

https://svelte.dev/playground/hello-world?version=5.56.4#H4sIAAAAAAAAE21RO0_DMBD-K8epQypQK9aQBCEWFiZGzJA6F2rVsYN9oVRR_juXB6CiDpbvPn-Ps92jKxvCFJ_IWg9HH2wFCVWGqVrjDdbGUsT0tUc-tSNvBARfVA9tu4mfZHnEdmWkS7j2jsmx2GAWdTAtF8opVs40rQ8MPdS2i_uXk9MwQB18AwpntcI7oQrZEkOEHFaRS6ZEYalwvZytqK5J86YNlCRryAvoJ3-W4Ogtbax_F8XMSsUbriFOYh5-PD466ujZ6OC5jIdzmzFX4W6eRfrfaZN_JnXnNBvvYNcxe_dojT5QJV4X5zkjLbdZ0mTlY6aWWe9ll2NI535OHPOy7d9jumx_W0xfeJVtpRRgtgfv9JiQ92dxQzEVp2w7wyKQj2L6Ykw5dDS8SVcaezSuwrQubaThG8vMaWIrAgAA

Logs

System Info

System:
  OS: macOS 15.7.7
  CPU: (10) arm64 Apple M1 Max
  Memory: 409.16 MB / 64.00 GB
  Shell: 5.9 - /bin/zsh
Binaries:
  Node: 24.14.1 - /Users/jace/.nvm/versions/node/v24.14.1/bin/node
  npm: 11.11.0 - /Users/jace/.nvm/versions/node/v24.14.1/bin/npm
  Deno: 2.8.2 - /opt/homebrew/bin/deno
Browsers:
  Brave Browser: 150.1.92.138
  Chrome: 145.0.7632.117
  Firefox: 128.0
  Safari: 26.5
npmPackages:
  rollup: ^4.0.0 => 4.61.1
  svelte: ^5.0.0 => 5.56.3

Severity

annoyance