QWidget::createWindowContainer does not work on 6.5.1

Using the linked test project, here is the result on desktop platform (6.5.1 or 6.5.0 behave exactly same):

When compiling for WebAssembly, using 6.5.0, behavior is the same:

But when using WebAssembly with 6.5.1, we have a sub-window which is created:

Which we can move around.

Note that it reset to it's default place / position on firefox window resize.

Expected behavior:

• on 6.5.1, the sub-window should not have decorations