Support for letter-spacing

[maxhoffmann]

Thank you for creating dropflow! It looks really cool. 🙂

I was wondering if you are also planning to add letter-spacing at some point? It’s the one essential typographic feature I’m currently missing.

[chearon]

Thank you! I'd love to add letter-spacing support. I think it's simple: walk the glyph array and increase advances at the end of each unicode grapheme:

This property specifies additional spacing (commonly called tracking) between adjacent typographic character units. Letter-spacing is applied after bidi reordering and is in addition to kerning and word-spacing. [CSS-WRITING-MODES-4] [CSS-FONTS-3] Depending on the justification rules in effect, user agents may further increase or decrease the space between typographic character units in order to justify text.

For the purpose of letter-spacing, each consecutive run of atomic inlines (such as images and inline blocks) is treated as a single typographic character unit.

Letter-spacing must not be applied at the beginning of a line. Whether letter-spacing is applied at the end of a line is undefined in this level.

— CSS Text §7.2

[chearon]

I did more reading and thinking on this. It's harder than I thought because the spacing has to be applied after bidi reordering. But letter-spacing also needs to be followed when measuring text for fit. If bidi text splits an inline, the common ancestor's letter-spacing is used. It sounds like I'm supposed to do a simple test of "will this split an inline", rather than trying to calculate the "hypothetical" bidi order.

Currently, none of the major browsers do this correctly: test.

LineWidthTracker, ShapedItem.measure, and createLineboxes all need to be updated to have letter spacing state. Those use logical order, but the total number of spaces will be the same as the after-bidi letter spacing. Also ShapedShim will need to gain an advance property since inline-blocks and <img> in the future also get spaces after them through letter-spacing.

[chearon]

Further reading: w3c/csswg-drafts#10193

[BobPritchett]

FWIW, it sounds like 'continuing with browser implementations' has won, with possible flags for the future alternate implementation. So just adding to the advance width, like most browsers, would be useful and meet expectations.

[chearon]

It's too bad there's been no movement on w3c/csswg-drafts#10193. Going with current browser behavior is fine, as long as the advance goes on the left side for RTL text, the way Firefox does. Some of the proposals in that issue sound way better than what browsers are doing, but it's probably best to wait until that gets resolved.

I'd still like to get the BIDI test to work in dropflow even though browsers don't do that either.