emoji: Use directional alignment for Unicode emoji on iOS

[yash-agarwa-l]

On iOS and macOS, use AlignmentDirectional.centerStart instead of Alignment.centerLeft for the Stack containing Unicode emoji.

This addresses a issue identified by @gnprice in this comment.

Before

LTR

RTL

After

LTR

RTL

[chrisbobbe]

I'm not seeing a difference between the before and after screenshots in RTL; are those the intended screenshots?

[yash-agarwa-l]

Yes, the changes made no visual difference.

[gnprice]

Hmm, interesting.

Does this alignment argument have any effect here? It sounds like it might not. And looking closer, I think that fits with the code: the Stack has only one non-positioned child, and I think Stack.alignment may only be relevant for aligning multiple non-positioned children to each other. The positioned child takes care of its own alignment.

… Ah, no, there's also this:

  /// Partially-positioned children, those that do not specify an alignment in a
  /// particular axis (e.g. that have neither `top` nor `bottom` set), use the
  /// alignment to determine how they should be positioned in that
  /// under-specified axis.

and the positioned child here doesn't specify its vertical alignment.

So the horizontal axis of this alignment argument has no effect, but we do need the vertical axis to be centered. Maybe Alignment.center is the cleanest thing to put here.

[yash-agarwa-l]

Thanks! That’s right, I’ve updated it to use Alignment.center.

[gnprice]

Thanks for the revision! Looks good; merging, once CI passes after this rebase.