Crash rendering a rare Chinese character

[kerams]

Describe the bug

Some built-in iOS font (both physical device and iPhone 16 iOS 18.5 simulator) doesn't align with format assumptions in Avalonia's font processing, causing a hard crash when looking for fallback fonts for an unusual character. On Android the character is not rendered at all. Not ideal (a placeholder would've been better), but at least the application continues to work.

To Reproduce

Add <TextBlock FontSize="24" TextWrapping="Wrap">e 𪜶</TextBlock> to SafeAreaDemo/MainView.xaml, launch, get InvalidOperationException("No suitable cmap subtable found.")

Expected behavior

Gracefully handle such typeface loading failures, and use a placeholder for 𪜶 if no glyph is found,

Avalonia version

master

OS

iOS 17.7, 18.5

Additional context

No response

[Gillibald]

This is not about a missing glyph. The font format is not supported at all. The question is what format is being used. It must be some ancient font.

[kerams]

If I had to guess, my money is on the last resort fallback format 13 (https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6cmap.html). Based on the description I don't think it's necessarily obsolete, so 13 (and 14?) should probably be supported.

[Gillibald]

Type 13 cmaps are only useful for fonts which use the same glyph for all the characters in a Unicode block—that is, only a Last Resort-like font will ever be likely to need one.

Format 13 seems to be Format 12 with some slight adjustment. Should be trivial to add.