From da794c4b365317d0e522fb9d343db6f7677a5961 Mon Sep 17 00:00:00 2001
From: Valentin Yushkevich <valiantsinyushkevich@gmail.com>
Date: Wed, 14 Oct 2026 14:58:16 +0300
Subject: [PATCH 1/2] fix: support attachTo on Vue versions before 3.5

app.onUnmount, used since #2700 to remove the attachTo container, only exists since Vue 3.5, so mounting with attachTo threw on older versions while the peer range is vue 3.x. Wrap app.unmount instead.
---
 src/mount.ts        | 18 ++++++++++++------
 tests/mount.spec.ts | 21 +++++++++++++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/src/mount.ts b/src/mount.ts
index 04d78c868..172390eb1 100644
--- a/src/mount.ts
+++ b/src/mount.ts
@@ -100,13 +100,19 @@ export function mount(
     }
 
     to.appendChild(el)
-    app.onUnmount(() => {
-      // The wrapper may already have been manually unmounted before auto-unmount
-      // runs, so the container can already be removed from its parent.
-      if (el.parentNode === to) {
-        to.removeChild(el)
+    // wrap app.unmount, as app.onUnmount is only available since Vue 3.5
+    const unmount = app.unmount
+    app.unmount = () => {
+      try {
+        unmount()
+      } finally {
+        // The wrapper may already have been manually unmounted before auto-unmount
+        // runs, so the container can already be removed from its parent.
+        if (el.parentNode === to) {
+          to.removeChild(el)
+        }
       }
-    })
+    }
   }
   const vm = app.mount(el)
   if (errorsOnMount.length) {
diff --git a/tests/mount.spec.ts b/tests/mount.spec.ts
index 534771003..619c68908 100644
--- a/tests/mount.spec.ts
+++ b/tests/mount.spec.ts
@@ -118,4 +118,25 @@ describe('mount: general tests', () => {
     wrapper.unmount()
     expect(document.body.firstChild, 'Container should be empty').toBeNull()
   })
+
+  // app.onUnmount only exists since Vue 3.5
+  it('supports attachTo without app.onUnmount', () => {
+    const wrapper = mount(Hello, {
+      props: { msg: 'Hello world' },
+      attachTo: document.body,
+      global: {
+        plugins: [
+          {
+            install(app: any) {
+              app.onUnmount = undefined
+            }
+          }
+        ]
+      }
+    })
+    expect(document.body.firstChild).toBeInstanceOf(HTMLDivElement)
+
+    wrapper.unmount()
+    expect(document.body.firstChild).toBeNull()
+  })
 })

From 35d458b165a1a134b47b2be6f5333821f4153e33 Mon Sep 17 00:00:00 2001
From: Valentin Yushkevich <valiantsinyushkevich@gmail.com>
Date: Wed, 21 Oct 2026 06:55:57 +0300
Subject: [PATCH 2/2] fix: use app.onUnmount when available

---
 src/mount.ts | 27 +++++++++++++++++----------
 1 file changed, 17 insertions(+), 10 deletions(-)

diff --git a/src/mount.ts b/src/mount.ts
index 172390eb1..917489fbe 100644
--- a/src/mount.ts
+++ b/src/mount.ts
@@ -100,16 +100,23 @@ export function mount(
     }
 
     to.appendChild(el)
-    // wrap app.unmount, as app.onUnmount is only available since Vue 3.5
-    const unmount = app.unmount
-    app.unmount = () => {
-      try {
-        unmount()
-      } finally {
-        // The wrapper may already have been manually unmounted before auto-unmount
-        // runs, so the container can already be removed from its parent.
-        if (el.parentNode === to) {
-          to.removeChild(el)
+    const removeContainer = () => {
+      // The wrapper may already have been manually unmounted before auto-unmount
+      // runs, so the container can already be removed from its parent.
+      if (el.parentNode === to) {
+        to.removeChild(el)
+      }
+    }
+    if (app.onUnmount) {
+      app.onUnmount(removeContainer)
+    } else {
+      // app.onUnmount is only available since Vue 3.5
+      const unmount = app.unmount
+      app.unmount = () => {
+        try {
+          unmount()
+        } finally {
+          removeContainer()
         }
       }
     }