Modal needs a way to disable the title bar

[chrisglein]

Summary

Modal is newly supported in Fabric, replacing Flyout and Popup from Paper (#11921). It uses a full child window to host UI, enabling moving and resizing and all sorts of things. Which... are not always desirable (#14646). If you're just trying to do an in-app popup, the extra Window chrome and features on Modal are not what you want. If you're trying to migrate an in-app flyout/popup from Paper, you're going to have a rough time.

At some point title was added (here #14636? Of note, was property documentation added for this, because it isn't an RN core property). What we need is titlebar styling to pick and choose what features show up.

When we looked at how to do Modal for RNW Fabric, we considered whether some of the existing properties would be right for this. For example presentationStyle (currently iOS only, but maybe relevant). Or transparent, which seems to have a real meaning closer to "fullscreen".
@TatianaKapos , do you happen to have that doc / write up of what we considered?

TLDR, we need props to configure the behavior of Modal on Windows. It likely will need additional properties (as this is an area where desktop will eclipse mobile's needs), but where we can logically adapt existing properties that's a good place to start.

Motivation

Currently there is no clear way to do "lightweight" popups in RNW Fabric.

Basic Example

No response

Open Questions

No response

[TatianaKapos]

Write-up here (can access with your microsoft account.

At the bottom of the write-up we did define a method that allowed the user to define the titlebar/remove it