Vue 3.5.44: reactivity, Suspense and transition fixes worth reading before you upgrade

Vue 3.5.44 shipped on 13 October and is marked as the latest stable release. It follows 3.5.43 (17 September) and lands while the 3.6 line is still in release-candidate mode, so the 3.5 branch is still where production fixes go. The changelog lists bug fixes only, 25 of them. Most are edge cases, but a handful change behaviour that real code can depend on, and those deserve a look before you bump the lockfile.

Reactivity: three behaviour changes

These are the entries I would read twice.

Watcher callbacks no longer track dependencies when run inside another effect (#15782, closes #5009). The changelog describes it only in those terms. The likely reading: reads inside a watcher callback that runs within an outer effect should stop being tracked by that outer effect. Code that accidentally relied on the extra tracking may stop re-running, so test nested watcher cases.

A once watcher now stops when its callback throws (#15755). The changelog gives only the one-line title, so check the PR for details. The implication is that a throwing callback now ends the watcher's life:

import { ref, watch } from "vue";

const token = ref<string | null>(null);

watch(
    token,
    (value) => {
        if (!value) return;
        parseClaims(value); // may throw
    },
    { once: true },
);

If you used once as "run the first time this is truthy, retry on failure", don't count on a retry after a throw. Catch inside the callback if you need to keep going.

Native array iterators are no longer mutated (#15703, closes #15643) and shallow collection semantics are preserved (#15793, closes #15792). Both are about the proxies reactivity hands out. If you have shallowReactive or shallowRef collections, re-run the tests that touch them.

Compiler and SFC changes

v-html now renders 0 and false as strings (#15571). Audit any v-html binding that can be 0 or false and confirm the output is what you want.
The v-memo cache guard wraps the v-for key in parentheses (#15724, closes #15723). That is a compile-output fix for keys that are expressions.
The SFC compiler reports an error when defineModel options reference a destructured prop (#15604, closes #15602). This one can break a build that used to pass:

vue

<script setup lang="ts">
const { min = 0 } = defineProps<{ min?: number }>();

// Now a compile error: the options reference a destructured prop.
const model = defineModel<number>({ default: min });
</script>

The changelog only says the compiler now reports an error here, so a build that used to pass may fail. Use a literal default, or derive the value after the model exists.

v-show gets its persisted transition injected in production builds (#15783).

Runtime-core: queue and props

Two fixes touch the scheduler and the patch path:

Only jobs queued by the root render itself are flushed (#15757, closes #6728). The flush is now scoped to the render's own jobs, so don't rely on a render flushing unrelated queued work.
Full-props comparison now detects new prop keys (#15634, closes #15632). The fix targets cases where a newly added prop key went unnoticed.

Also in the list: an invalid v-for range renders nothing in production (#15742), and the async placeholder is kept for block patching and pending component updates (#15554, closes #15551).

Suspense, hydration, Teleport and transitions

This is the bulk of the release and the area that has been getting steady fixes since 3.5.42 (27 August):

Suspense: a resolved async component is mounted with its own namespace (#15640), and async components that are unmounted while pending are no longer rendered.
Hydration: hydrateOnVisible now observes children of a display: contents root (#15773).- Teleport: targetStart is moved and children stay before targetAnchor (#15818, closes #15817).
Transitions: comment-wrapped component roots are handled (#15853, closes #6080, #5500), and nested TransitionGroups are measured against their moving outer item (#15848, closes #5385, #1531).
KeepAlive: a pruned current component is unmounted when its vnode was cloned on mount (#15711).
Custom elements: an async custom element removed while loading is no longer mounted (#15777).
SSR: async setup errors propagate from the server renderer (#15857, closes #15716) and falsy vnode content overrides are honoured (#15769).

Server-side error propagation is the one to notice for Nuxt and other SSR apps. Async setup errors now propagate from the server renderer, so watch your logs and error pages after upgrading.

Smaller items: playsinline is a boolean attribute (#15627), v-show keeps its display value for unchanged style strings (#15794), and the Vue 2.7 compat build exposes _setupProxy (#15618).

How to upgrade

This is a patch release, but the reactivity and compiler entries are behaviour changes. A sensible order:

Bump and run the type check and build first. The defineModel error will show up there rather than at runtime.
Grep for once: true watchers and for watch callbacks that read state an outer computed also uses.
Check any v-html binding that can be 0 or false.
Watch the server logs on the first deploy for newly surfaced async setup errors.

Trade-offs

Patch releases that tighten semantics are the right call, but they are not free. Fixes like the watcher-tracking one remove accidental coupling, and code that depended on the coupling breaks without an error. Nothing in the changelog is labelled breaking, so the safety net is your own tests, not the version number. If your suite lacks coverage around watchers nested in computed or render effects, add a test before upgrading rather than after.

Nothing here changes the 3.6 story. The 3.6 release candidates are still the place for Vapor work; 3.5.44 is for apps that stay on the stable renderer.