[fix]: make WorkflowHostingController background color clear

[n8chur]

Note: This PR is targeting the feature/viewenvironmentui branch and depends on #218.

Overview

This PR is part of the larger effort to add support for automatic ViewEnvironment bridging between UIKit propagation and WorkflowUI propagation within WorkflowUI.

See the associated proposal for more information.

While this change may seem unrelated to the view environment bridging work, it was added to this feature branch since it allows replacement of DescribedViewController usages with WorkflowHostingControllers without loading the view earlier than you may have needed to previously. WorkflowHostingController rendering a single screen is one way to achieve automatic bridging of the ViewEnvironment from a vanilla propagation path to a Screen which can be useful in contexts like snapshot testing harnesses. While we may want to provide an automatic bridging container specific to Screen's, this work is currently considered out of scope for this feature branch (although that's open to reconsideration)—see the this section in the proposal for more information on this topic.

DescribedViewController's backgroundColor is .clear, which is an intuitive value when considering that it should appear identical to the content it is wrapping—it shouldn't place an opaque .white background underneath. I'd argue the same applies to WorkflowHostingController.

Today, if you want to set the WorkflowHostingController's backgroundColor to .clear, you'd need to load the view first (workflowHostingController.view.backgroundColor = .clear)—there is no way override the background color otherwise. Loading the view early like this can be problematic in some cases, e.g. when attempting to utilize a WorkflowHostingController in snapshot testing infrastructure where you may want to build up a UIViewController hierarchy before loading the view at all in case it references values in the vanilla ViewEnvironment propagation path which need to be mutated above it.

Square Integration PRs

https://github.com/squareup/market/pull/6286
https://github.com/squareup/ios-register/pull/86301

Checklist

• Unit Tests
• UI Tests
• Snapshot Tests (iOS only)
• I have made corresponding changes to the documentation

[square-tomb]

Is it possible that some of the sample apps in Samples will become illegible with this change?

[square-tomb]

This probably should have been .backgroundColor in the first place 🤔

[robmaceachern]

🎶 I can see clearly now 🎶

[n8chur]

Is it possible that some of the sample apps in Samples will become illegible with this change?

It's possible! I'll browse through them and double check.

• Validate that sample app backgrounds still look good

[watt]

Not sure if you're keeping track elsewhere, but make sure this gets into the changelog!

[n8chur]

make sure this gets into the changelog!

I don't think the changelog within the repo is maintained, is it? It looks like it's generated for the release description manually based on PRs.