Support `text-indent`

[dolsup]

Parley doesn't currently have a way to indent the first line of a paragraph (like CSS text-indent).

I tried working around this by inserting a zero-height InlineBox at the start of the paragraph as a spacer. This mostly works, but the line breaker unconditionally marks a break opportunity after every inline box:

parley/parley/src/layout/line_break.rs

Lines 257 to 258 in df7cedc

	// We can always line break after an inline box
	self.state.mark_line_break_opportunity();

With these settings:

builder.push_default(StyleProperty::OverflowWrap(OverflowWrap::Anywhere));
builder.push_default(StyleProperty::WordBreak(WordBreakStrength::BreakAll));

Filling a line with , characters causes the InlineBox to be orphaned on a line by itself — the text moves to the next line at x=0 with no indent. (This doesn't happen with A characters.)

A native text_indent parameter would avoid this problem. Negative values could give hanging indents, useful for list markers.

[devunt]

Hi, I'm on the same team as @dolsup. We're building a Rust-native rich text editor using parley and need support for CSS text-indent. As you can see in #550, we had been using parley's InlineBox to achieve text-indent, but that was a workaround.

We're ready to contribute a proper text-indent implementation to parley, but there's one design decision we'd like to align on first. Currently, parley doesn't have an explicit concept of a paragraph. Per the CSS spec, text-indent applied to a paragraph should only affect the very first line of that paragraph — it should not be re-applied after soft wraps or even after hard breaks (<br/>). In other words, indentation should only appear on the first line of the element to which text-indent is applied, regardless of what kind of line break occurs within it.

Given that parley has no paragraph concept, my expectation would be that if the text being laid out contains hard breaks (newline, \n), indentation should not be re-applied on the lines following those breaks — consistent with the CSS spec. Would it be okay to proceed with the implementation based on this assumption?

The caveat of this approach is that laying out multi-paragraph text with text-indent in a single parley context would not be possible; each paragraph would need to be laid out in its own separate parley context. In our use case this isn't an issue, since we already lay out each paragraph element separately (for layout caching purposes, among others). That said, I'd love to hear your thoughts on this, @nicoburns.

[nicoburns]

We're building a Rust-native rich text editor using parley and need support for CSS text-indent.

That's cool! Is your rich text editor open source? (if it is, then I would be very interested to see the code, but if not then nevermind).

We're ready to contribute a proper text-indent implementation to parley, but there's one design decision we'd like to align on first.

Amazing!

Given that parley has no paragraph concept, my expectation would be that if the text being laid out contains hard breaks (newline, \n), indentation should not be re-applied on the lines following those breaks — consistent with the CSS spec. Would it be okay to proceed with the implementation based on this assumption?

This is an interesting question. I've done a little bit of research and found that the CSS spec has the each-line modifier to control this. I would suggest also introducing this to Parley either as an enum. Something like:

enum TextIndentScope {
     FirstLayoutLine,
     EachLine,
}

You could also consider implementing the hanging variants, where the indent is inverted (but we can also add this later).

See:

• MDN https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/text-indent
• Spee https://drafts.csswg.org/css-text/#text-indent-property

The caveat of this approach is that laying out multi-paragraph text with text-indent in a single parley context would not be possible; each paragraph would need to be laid out in its own separate parley context. In our use case this isn't an issue, since we already lay out each paragraph element separately (for layout caching purposes, among others).

Blitz also takes this approach of using a separate Layout for each paragraph (we are laying out HTML, so we map block elements to parley Layout and <br /> to newline). I believe Xilem may currently just use newlines for paragraphs.

[waywardmonkeys]

This feels like the type of thing that we're trying to not do in Parley in this way (especially with respect to Parley Core).

I think we need feedback from @dfrg

[nicoburns]

This feels like the type of thing that we're trying to not do in Parley in this way (especially with respect to Parley Core).

It certainly wouldn't be in Parley Core (the purpose of which is not to be opinionated about layout). But then I wouldn't expect the Layout type to exist at all in Parley Core. I would expect any line-breaking functionality would likely be much lower-level iterator style and be agnostic to storage.

But I feel like it probably ought to be in Parley. Isn't that the whole point of splitting the crates up to have one that's unopinionated low-level primitives (parley core) and one (or more) that provides a fully-featured out of the box experience (parley)? I'm very puzzled by this recent attitude of opposition to adding features to Parley. It's not like text indentation is a css-specific feature. It's a very common requirement for anything that's laying out prose.

I would add, that the implementation submitted (#551) seems quite unobtrusive to me. It's +63 -6 LoC (ignoring tests), and it is essentially implements text-indentation as a modifier on alignment