Add regression tests for content measured at the unclamped style width

[HelgeSverre]

Objective

Pin down a failure mode that #989 fixed incidentally but left untested: a flex child whose style width resolves wider than its own max_width (width: 100% of a wider parent, or a fixed width above the cap) used to have its content measured at the unclamped width while being laid out at the clamped one. With a measure function whose height depends on its width (wrapped text), the child's flex base size went stale — the child and its ancestors ended up shorter than the content laid out inside them.

On 0.12.2 and earlier this makes downstream UIs paint blocks on top of each other: gpui/Zed's long-standing "overlapping text" bugs (zed-industries/zed#30002, zed-industries/zed#30097, worked around in-app in zed-industries/zed#30377) reduce to exactly this — gpui's virtualized list stacks items by their root height, and a width: 100%; max-width: N readable column inside an item left the root ~1 line short per wrapped paragraph.

Minimal standalone repro — fails on 0.12.2 (`column: 560x40` vs text `560x60`), passes on 0.13.0

use taffy::prelude::*;
use taffy::AvailableSpace;

/// A wrapping text: 1163px long unwrapped, 20px per line.
fn measure_text(known: Size<Option<f32>>, available: Size<AvailableSpace>) -> Size<f32> {
    const TEXT_WIDTH: f32 = 1163.0;
    const LINE_HEIGHT: f32 = 20.0;
    let width = known.width.or(match available.width {
        AvailableSpace::Definite(w) => Some(w),
        _ => None,
    });
    match width {
        Some(w) => {
            let lines = (TEXT_WIDTH / w).ceil().max(1.0);
            Size { width: w.min(TEXT_WIDTH), height: lines * LINE_HEIGHT }
        }
        None => Size { width: TEXT_WIDTH, height: LINE_HEIGHT },
    }
}

fn main() {
    let mut tree: TaffyTree<()> = TaffyTree::new();
    let text = tree.new_leaf_with_context(Style::default(), ()).unwrap();
    let column = tree
        .new_with_children(
            Style {
                flex_direction: FlexDirection::Column,
                size: Size { width: percent(1.0), height: auto() },
                max_size: Size { width: length(560.0), height: auto() },
                ..Default::default()
            },
            &[text],
        )
        .unwrap();
    let root = tree
        .new_with_children(
            Style {
                flex_direction: FlexDirection::Column,
                size: Size { width: length(700.0), height: auto() },
                ..Default::default()
            },
            &[column],
        )
        .unwrap();
    tree.compute_layout_with_measure(root, Size::MAX_CONTENT, |known, available, _, _, _| {
        measure_text(known, available)
    })
    .unwrap();
    let column_size = tree.layout(column).unwrap().size;
    let text_size = tree.layout(text).unwrap().size;
    println!("column: {}x{}", column_size.width, column_size.height);
    println!("text:   {}x{}", text_size.width, text_size.height);
    assert!(column_size.height >= text_size.height, "column is shorter than the text inside it");
}

The clamp added in #989 (motivated by aspect-ratio transferred sizes) also covers plain min/max constraints at the flex-basis site, which fixes this class — but nothing exercised the measure-function/wrapped-text consequence, so it could silently regress.

For conformance context: Chrome lays out the analog DOM (width: 100%; max-width: 560px around wrapping text) with the container containing its wrapped text, and flexbox §9.2 step 3 item E sizes the item at its used cross size (the "min and max sizes are ignored" exemption at flex-basis time is main-axis only).

Changes

Two generated HTML fixtures in test_fixtures/flex, using Ahem text whose line count differs between the unclamped and clamped widths (2 lines at 300px vs 3 lines at 200px):

• measure_child_with_percent_width_clamped_by_max_width — width: 100% of a 300px parent with max-width: 200px; the text and containing column are 200px × 30px.
• measure_child_with_fixed_width_clamped_by_max_width_intrinsic — the same clamp through the intrinsic-sizing path with a content-sized root.

The generator renders both fixtures in Chrome and emits border-box/content-box and LTR/RTL variants, so all expected layouts are browser-derived.

Test results

• cargo test --test xml: 5549 passed, 0 failed.
• cargo test --tests: passed (4 pre-existing ignored tests).
• cargo +nightly clippy --workspace: passed.
• Fixture formatting and a second just gentest run produced byte-identical generated output.
• The percentage-width fixture fails on Taffy 0.12.2 with 20px ancestors around a 30px text leaf, and passes on the PR branch with all three heights at 30px.

[nicoburns]

Thanks! But could we make these generated tests please? Generated tests automatically assert their layout against Chrome, so don't need to trust/verify their assertions and they are easier to maintain.

See CONTRIBUTING.md

[HelgeSverre]

I think i did it right, let me know if this is what you meant @nicoburns

[HelgeSverre]

@nicoburns feel free to close this if you merged a superseding version in already.

[nicoburns]

Ack, I thought I had merged this, but I guess the auto-merge never triggered.