[Fabric] Modal opens a new window on visible

[TatianaKapos]

Description

When visible is set to true, Modal creates a new window with the root hwnd as the parent window.

Type of Change

• New feature (non-breaking change which adds functionality)

Why

Currently Modal does nothing, now Modal does something! Incrementally adding small changes to Modal but looking for feedback if this is the right direction we want to go in :)

Next Steps for Modal documented here: #11157. Next step is to add react native components within the new window

What

Creates and register a new window for Modal. Currently you can only destroy the window by manually pressing the 'X' button as opposed to setting visible to false.

Screenshots

Playground-composition_xs1HVgun38.mp4

Testing

tested locally

Changelog

no

Microsoft Reviewers: Open in CodeFlow

[chrisglein]

Is WINRT_VERIFY a debug-only assert? looks like no
Why do we have both checks here?

check_win32 takes in a... success code? reference. But this an ATOM. Doesn't look like it does the work of calling GetLastError? Basically... I'm not sure this is the right error checking.

[TatianaKapos]

huh interesting! I'm not sure on the right error checking for an ATOM but I can do some research! (unless someone else knows)

Doing a quick search of our repo, https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/microsoft/react-native-windows%24+RegisterClassEx&patternType=keyword&sm=0, it looks like this is how we check for an error when calling RegisterClassEX in multiple place, wondering if we will also want to change these?

[jonthysell]

Can you refactor this slightly into two functions, something like:

1. EnsureModalCreated() which does the work of checking m_hwnd and building the window
2. ShowOnUIThread() which calls that function then calls ShowWindow, etc.

I think we'll run into other places where we'll want to create the window so it's ready for UI, even if we don't show it right away.