[ValentinYoushkevich]

mount, shallowMount and renderToString install their stubs with Vue's global transformVNodeArgs and never remove them, so any other app rendered later in the same file still gets stubbed, even after unmount():

shallowMount(Loading).unmount()

const el = document.createElement('div')
createApp({ render: () => h(Loading) }).mount(el)
el.innerHTML // '<b-loading-stub></b-loading-stub>'

The same happens with createSSRApp + @vue/server-renderer (<transition-stub> in the server HTML, hydration mismatch), and a wrapper mounted earlier re-renders with the stubs of the last mounted one. Repro: playground (2.5.1), playground (main)

The transformer of each app is now stored in a WeakMap keyed by the app, and a single dispatcher installed with transformVNodeArgs applies the one of the app being rendered (vnodes created outside of any component still use the last mounted wrapper's transformer). Since every transformer now only sees its own app, the mountedRootComponents set from #2890 is no longer needed and is removed (the #2675 test still passes). Note that components rendered by another app, e.g. one created with createApp inside a component, are no longer stubbed.

Projects working around this in their tests:

• buefy: Loading.spec.ts#L72-L75, Toast.spec.ts#L37-L39, Modal.spec.ts#L109-L111
• wikibase-termbox (SSR): InMoreLanguagesExpandable.spec.ts#L47-L48
• ELA-Bot: MainLayout.transition.test.ts#L8-L13

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