keepMountedMode for components other than Tabs (e.g. Dialog)

[mfulton26]

I ran into the same issue I ran into previously in https://github.com/orgs/mantinedev/discussions/8832 but with <Dialog>. I'm already using keepMounted but the initial render has opened={false} and so nothing ever gets loaded, but I need it to load hidden in order for other things in the UI to work (e.g. loading an iframe that other code interacts with).

Can keepMountedMode also be added to <Dialog>? Would it be appropriate to add it to all components that use React 19's <Activity>? The ability to set a default might be helpful too (e.g. via something like <MaintineProvider> props).