Guess glyph string for placeholder glyphs without code points

[khaledhosny]

See the discussion in #2005

This works in the font overview, but I didn’t manage to find the relevant code in editor, so editing such glyphs still does not shows blank placeholder.

[justvanrossum]

Thank you very much!

I wonder if the same can be achieved with the "associatedCodePoints" feature. This does roughly the same (including the "_" split), except it also takes the font's cmap/glyphMap into account, and only falls back to glyphs-data if the font doesn't define the base glyph name.

I think we can add associatedCodePoints as an (optional?) argument to the GlyphCell constructor.

Alternatively, we could consider decoupling the making of the placeholder string from the GlyphCell completely, and pass the placeholder string as an additional argument to the GlyphCell constructor.

As for the placeholder string in the canvas: let's indeed focus on the GlyphCell first, and I'll try to figure out what we need to do to make it work in the canvas, too.

[khaledhosny]

I wonder if the same can be achieved with the "associatedCodePoints" feature.

The main difference between associatedCodePoints and this placeholder string is that I insert ZWJ before and/or after the code point to force an Arabic positional form.

Take for example the four glyphs beh-ar, beh-ar.init, beh-ar.medi, and beh-ar.fina. The associatedCodePoints for the last three would be the code point for beh-ar, so using this would show the same fallback for all the four glyphs. Instead, I insert ZWJ after the code point in .init to force initial form, before the code point in .fina to force final form, and before and after the code point in .medi to force the medial form.

Is there is a way to achieve this with associatedCodePoints?

[justvanrossum]

(Thanks for keeping this up to date, I'll try to continue reviewing this soon. I am eager to land this feature.)

[justvanrossum]

This is looking very good, thank you. I left some minor notes, and a suggestion how to maybe fix the context.direction = "rtl"; line.