Adding safe area edge detection for the scrollableAxes behavior

[johnnewman-square]

Adding a ScrollView API to allow clients to specify the edges that should be inset for the safe area when using the scrollableAxes content inset behavior. Before this change, content that landed outside the safe area but inside the scroll view bounds was not scrollable when using scrollableAxes. (See linked docs for details)

The goal of this update is to address the issue of content not being scrollable without introducing breaking API changes or large behavioral changes to ScrollView. A few alternative approaches could be:

• turn on alwaysBouncesVertical and/or alwaysBounceHorizontal, which will cause scrollableAxes to honor the safe areas along both edges of the bouncing axes.
• update client implementations to use the always content inset behavior. This would require refactoring the ScrollView layout to no longer produce a contentSize equal to the ScrollView bounds when using fittingWidth and fittingHeight.

The approach in this PR will enable the safe area behavior we need along a precise edge, like the bottom, while limiting the risk of regressions.

[watt]

Very thorough and well-documented, nice work!

[watt]

Is this meant to be used somewhere, or just cruft?

[johnnewman-square]

This property is used to minimally apply the .bottom edge if the client hasn’t already set an edge, in this PR. If the client already configured the edge behavior, this property is used to avoid making further changes.

[johnnewman-square]

After chatting in person, we discussed defaulting scrollableAxesSafeAreaEdges to nil, which we could then check against to determine if the client has modified the property.

(In the linked PR, we can ask those clients to supply the empty set if they want to opt out of the .bottom inset behavior.)

• Remove didModifyScrollableAxesSafeAreaEdges and make scrollableAxesSafeAreaEdges default to nil.

[watt]

Is there any reason not to make the default .all?

[johnnewman-square]

Thanks for the suggestion. There were a few reasons that the empty set was used for the default:

• Specifying SafeAreaEdge.all will have the same effect as setting the contentInsetAdjustmentBehavior to .always.
• Including the .top edge breaks a number of snapshots that currently draw content within the top safe area. Similar to the snapshot breakages, .top can change expected behavior at runtime if a client aligned the ScrollView to the screen's top edge and manually adjusted its content to avoid the safe area by including padding around their elements.
• Including the horizontal edges can also break snapshots and will enable horizontal scrolling in contexts that traditionally have not horizontally scrolled, like cases where we’re using ContentSize.fittingHeight and constraining the ScrollView to the screen's left or right edge.

I'd like to provide this control knob to clients without enabling it automatically, with the exception of this change, where we’re minimally enabling the .bottom edge. Happy to chat further though!

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I can make .all internal and add a note explaining that clients should use ContentInsetAdjustmentBehavior.always instead:

• Update SafeAreaEdge.all