test: move beforeunload tests to main runner and fix flake (#18432)

spec/window-helpers.js

@@ -4,15 +4,29 @@ const { BrowserWindow } = remote
 
 const { emittedOnce } = require('./events-helpers')
 
+async function ensureWindowIsClosed (window) {
+  if (window && !window.isDestroyed()) {
+    if (window.webContents && !window.webContents.isDestroyed()) {
+      // If a window isn't destroyed already, and it has non-destroyed WebContents,
+      // then calling destroy() won't immediately destroy it, as it may have
+      // <webview> children which need to be destroyed first. In that case, we
+      // await the 'closed' event which signals the complete shutdown of the
+      // window.
+      const isClosed = emittedOnce(window, 'closed')
+      window.destroy()
+      await isClosed
+    } else {
+      // If there's no WebContents or if the WebContents is already destroyed,
+      // then the 'closed' event has already been emitted so there's nothing to
+      // wait for.
+      window.destroy()
+    }
+  }
+}
+
 exports.closeWindow = async (window = null,
   { assertSingleWindow } = { assertSingleWindow: true }) => {
-  const windowExists = (window !== null) && !window.isDestroyed()
-  if (windowExists) {
-    const isClosed = emittedOnce(window, 'closed')
-    window.setClosable(true)
-    window.close()
-    await isClosed
-  }
+  await ensureWindowIsClosed(window)
 
   if (assertSingleWindow) {
     // Although we want to assert that no windows were left handing around
@@ -22,9 +36,7 @@ exports.closeWindow = async (window = null,
     const windows = BrowserWindow.getAllWindows()
     for (const win of windows) {
       if (win.id !== currentId) {
-        const closePromise = emittedOnce(win, 'closed')
-        win.close()
-        await closePromise
+        await ensureWindowIsClosed(win)
       }
     }
     expect(windows).to.have.lengthOf(1)