Steps to reproduce:
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1. Enable "Automatic Receipt Printing" so the receipt is auto printed after paying an order
2. From PoS, make an order, select it to invoice, and click the pay button.
3. While loading (normally a few seconds to finish the invoice), switch the tab and stay there for few seconds (the time the invoicing has finsihed).
4. Come back to the initial page, a traceback will appear and the ticket is not printed.

Why it happens:
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The receipt is printed when the parent of the `OrderReceipt`, i.e. `RenderContainer` is rendered. In this case, we assume that `OrderReceipt` has already had the time to be mounted and thus it's in the DOM, so in this case, `this.ref?.el?.firstElementChild` has the order receipt. However, when switching the tab, and since OWL uses `requestAnimationFrame` as a scheduler, and since the browser will throttle `requestAnimationFrame` when the tab is not active, we will not have access to the order receipt component in its parent's `onRendered`!

The fix
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Instead of seeing the parent renders as a sign that its child has been successfully put in the DOM, we now renders the child component (OrderReceipt) in its own container and thus we can hook into its `onMounted` lifecycle where we know for sure that this component has been successfully mounted and attached to the DOM, and can be accessed through `this.ref?.el?.firstElementChild`.

opw-5124585

Forward-Port-Of: odoo/odoo#241781
Forward-Port-Of: odoo/odoo#239008