ModalOverlay: Keep content mounted when closed

### Provide a general summary of the feature here

Would it be possible to somehow keep content mounted when the modal is closed? We're working on a modal sheet with an iframe, and we would like to preload the iframe while the modal is closed.

### 🤔 Expected Behavior?

Prop that allows the ModalOverlay to stay mounted while closed

### 😯 Current Behavior

Content is unmounted when the modal closes.

### 💁 Possible Solution

_No response_

### 🔦 Context

Iframe in a sheet like modal

### 💻 Examples

_No response_

### 🧢 Your Company/Team

_No response_

### 🕷 Tracking Issue

_No response_

Start at the ModalOverlay component entry point and trace how its content is mounted when closed. Locate the relevant tests before editing, then add coverage for the requested prop and verify that content remains mounted while the modal is closed.

Written by the indexing model from the issue text.