Ligatures should be disabled when letter spacing is enabled

[taj-p]

As the title suggests, ligatures should be disabled when letter spacing is requested. Otherwise, the clusters that are sent to breaking represent ligatures that cannot be broken up.

For example, in the below test, we produce this image. Note how "ffi" in office are out of place and how the "fi" and "fl" ligatures are not broken up like "ff".

#[test]
fn interaction_letter_spacing_ligatures() {
    let mut env = TestEnv::new(test_name!(), None);
    let text = samples::LIGATURES;

    // Without letter spacing - ligatures should form (if font supports them)
    let features_on = FontFeatures::List(Cow::Borrowed(&[FontFeature {
        tag: Tag::new(b"liga"),
        value: 1,
    }]));

    let mut builder_no_spacing = env.ranged_builder(text);
    builder_no_spacing.push_default(StyleProperty::FontFeatures(features_on.clone()));
    builder_no_spacing.push_default(StyleProperty::LetterSpacing(0.0));
    let mut layout_no_spacing = builder_no_spacing.build(text);
    layout_no_spacing.break_all_lines(None);
    layout_no_spacing.align(None, Alignment::Start, AlignmentOptions::default());

    env.with_name("no_spacing")
        .check_layout_snapshot(&layout_no_spacing);

    // With letter spacing - ligatures SHOULD break
    let mut builder_with_spacing = env.ranged_builder(text);
    builder_with_spacing.push_default(StyleProperty::FontFeatures(features_on));
    builder_with_spacing.push_default(StyleProperty::LetterSpacing(2.0));
    let mut layout_with_spacing = builder_with_spacing.build(text);
    layout_with_spacing.break_all_lines(None);
    layout_with_spacing.align(None, Alignment::Start, AlignmentOptions::default());

    env.with_name("with_spacing")
        .check_layout_snapshot(&layout_with_spacing);
}

Seen in #503

[dfrg]

+1. I think there might be script specific considerations here as well. Arabic comes to mind because it essentially requires ligatures to be legible.

[behdad]

+1. I think there might be script specific considerations here as well. Arabic comes to mind because it essentially requires ligatures to be legible.

Yes & no. Arabic does have a couple of "required ligatures", but those should be done with rlig that stays on even if you turn liga off.

Arabic and some other scripts are "cursive" though, and they break if you letter-space them blindly. I think @khaledhosny might better know how different systems deal with such scripts.

In HarfBuzz we considered whether to add a function to flag such scripts, but we decided against it: harfbuzz/harfbuzz#64

[khaledhosny]

CSS Text Module specification of the letter-spacing property is a good resource on this. It is what (modern) web browsers (aspire to) implement. Many other systems apply tracking/letter spacing blindly.

[dfrg]

Ah yes, I forgot about rlig.

Looks like Chrome just doesn’t apply letter spacing for Arabic text. For Latin, Chrome disables ligatures entirely and I can’t find a way to override.

Safari applies letter spacing to Arabic. When non-zero letter spacing is enabled (with any script), ligatures are disabled and the font-variant-ligatures property is ignored but you can override with font-feature-settings.

I haven’t checked Firefox yet.

[yisibl]

Looks like Chrome just doesn’t apply letter spacing for Arabic text.

These two may be related.

• https://chromium-review.googlesource.com/c/chromium/src/+/6399436
• https://bugzilla.mozilla.org/show_bug.cgi?id=1342835

The behavior in Servo is slightly different. See: https://issues.chromium.org/issues/40618336#comment26