Enable floats and other advanced layouts by nicoburns · Pull Request #421 · linebender/parley

nicoburns · 2025-09-26T13:38:41Z

Related PRs

Depends on Compute whitespace properties and line metrics for each line eagerly #420
Depends on Add Cluster::from_point_exact method for hit-testing spans #447
Fixes Floated boxes (CSS Floats) #99
Fixes Make container_width a required alignment parameter #283

Introduction

This PR is aimed at enabling Blitz to implement CSS floats on top of Parley. However, the changes made are not float-specific, and will also enable other advanced layouts such as those that have "exluded regions" or want to lay out text into a complex shape.

Parley's existing architecture

While most users call the simple Layout::break_all_lines(max_advance) to perform line breaking, Parley does already have a lower-level Layout::break_lines() function which returns a BreakLines struct, on which you can call BreakLines::next(). Each call to BreakLines::next() lays out a single line and then returns. This PR extends the functionality of this method so that in addition returning when a line-break occurs, it will also return when:
- It encounters a floated box
- The configured max_line_height is exceeded
Parley's Line struct has min_coord and max_coord fields.

Changes Made

Per-line bounding box `max_advance`

When using the BreakLines struct directly, the max_advance (max width) can now be configured on a per-line basis. This makes it possible to create layouts with lines broken to different lengths. Additionally, it is valid to adjust the max_advance midway through line-breaking a line (it is advised to ensure that the existing content of the line still fits, although nothing terrible will happen if you don't (the contents will just overflow)).

Automatically determined (per-line) alignment width

Related to the per-line max-advances, the width used for alignment is now per-line and determined automatically and is equal to the max_advance for that line (at the time that the line is "committed"). This means that each line is aligned within it's own width bound, separately from the other liens. This is really the only way to sensibly align lines of differing widths, but making alignment width automatic had previously been discussed as being desirable anyway.

Added `max_line_height`

Each line now has a max_line_height. If at any point the line exceeds this height, control flow will be yielded from BreakLines::next(), allowing the user to find a new, larger space to lay out into. By default, max_line_height is set to f32::MAX and thus has no effect.

Customisable per-line x- and y-offset

Parley previously assumed that every line started at x=0 and that the start of the next line would be immediately below the previous line. Parley now allows an x and y offset to be manually configured for each line. This does not affect line breaking, but is used when reporting the positions of glyphs and for selection.

The x offset is in addition to and stored separately from the offset generated by alignment.

Added `InlineBoxKind` enum

A new kind: InlineBoxKind field has been added the InlineBox struct, where InlineBoxKind is defined as:

enum InlineBoxKind {
     InFlow,
     OutOfFlow,
     CustomOutOfFlow,
}

InlineBoxKind::InFlow represents the existing kind of inline box that is is laid out in-flow with text like a display: inline-block box in CSS.
InlineBoxKind::OutOfFlow is assigned a position during layout in exactly the same way as an InFlow box. However it does not take up space or affect the position of other items (glyphs or boxes) in the layout. This corresponds to a position: absolute box in CSS. Blitz was previously representing this as a zero-sized box, but I have taken the opportunity here to represent it explicitly.
InlineBoxKind::CustomOutOfFlow is the box kind for a floated box. Parley does not attempt to lay these out at all. When it encountered a box of this kind it yields control flow back to the caller who then responsible for positioning the box, adjusting the line's position/size, and then resuming layout (details below).

Tasks

Allow boxes to marked as in-flow or out-of-flow
Allow line-breaking to yield control flow when encountering a box
Allow users of parley to the x/y position and the max-width of each line
Update selection to account for line offsets
Update alignment to be per-line

taj-p

Sorry for the delay on the review - this PR is actually way more complex than I expected. Kudos on being able to wrangle the complexity of line breaking to support this custom behaviour.

taj-p · 2025-11-16T05:13:43Z

Can you please update this documentation?

I think this still needs a little bit of updating.

Added suggestion below

taj-p · 2025-11-16T05:26:40Z

+        if self.state.layout_max_advance >= f32::MAX {
+            self.layout.data.alignment_width = full_width;
+            for line in &mut self.lines.lines {
+                line.metrics.inline_max_coord = line.metrics.inline_min_coord + width;


For my information, why do we want to overwrite the inline_max_coord for a line with the maximum width + inline_min_coord? What utility does it provide?

Hmm... I don't think this is correct anymore actually. The basic logic of "if a width constraint is infinite, then set it to a computed value" (+ we treat f32::MAX as infinite) makes sense. But I think it should be done on a line-by-line basis rather than using the layout_max_advance now.

The key thing this is trying to avoid is Parley performing alignment against an infinite width. That not only positions text completely incorrectly, but also causes performance issues.

Ok, I've dug into this a bit. This handles the case where layout is called with a None or Infinite width constraint (and no line-specific constraints are set during the layout). In that case we'd like alignment to be relative to the width of the longest line.

It's a bit of an edge case (you probably shouldn't be calling layout with an infinite constraint), but I think this is a nice fallback behaviour for alignment in case you do.

taj-p

This very much LGTM - left some minor comments. Just want to ensure I understand everything.

When you've addressed the comments, hit the "re-request review" button so it shows up in my review queue and I PROMISE I'll review it the next business day

taj-p · 2026-04-04T20:12:40Z

Added suggestion below

taj-p · 2026-04-04T20:17:32Z

+    pub line_y_start: f64,
+    pub line_y_end: f64,


Do we need to specify somewhere that coordinates use physical x/y axes. For horizontal text, x corresponds to the inline direction and y to the block direction? Or is this captured elsewhere in Parley?

Um... big question! Perhaps something we can revist when we actually support vertical text? (he says hopefully)

SGTM!

he says hopefully

👀 From my understanding, I think this will be worked on in 2026

taj-p

Hey Nico! Just checking whether you saw the previous comments - if you disagree with them, that's fine. Just want to check that they weren't missed accidentally.

taj-p · 2026-04-10T07:19:14Z

-struct BreakerState {
+/// Reason that the line breaker has yielded control flow
+pub enum YieldData {
+    /// Control flow was yielded because a line break was encountered


Did we not want to adopt any of the below suggestions?

nicoburns · 2026-04-11T16:12:01Z

@taj-p I had missed a few things! I think I've got everything now.

taj-p

LGTM 🎉 !!!! Let's goo!! Nice work!! 🚀

Signed-off-by: Nico Burns <nico@nicoburns.com> Ignore out-of-flow boxes for the purpose of whitespace collapsing Rename coords to block Signed-off-by: Nico Burns <nico@nicoburns.com> Fixup tests (inline box kind) Allow the caller to set x/y/max_advance for each line Signed-off-by: Nico Burns <nico@nicoburns.com> Add advance to box yield Align each line to it's own max-width Don't include last line in layout's height if it's empty Fix Cluster::from_point to work with offset lines Signed-off-by: Nico Burns <nico@nicoburns.com> Remove alignment_width argument from tests Use inline min coord in more places Use layout_max_advance for rendering tests Accept new snapshots Signed-off-by: Nico Burns <nico@nicoburns.com> Fix max_advance of f32::MAX MaxLineHeight WIP Signed-off-by: Nico Burns <nico@nicoburns.com> Fix: only InFlow InlineBox's contribute size to the layout Make InlineBoxKind Copy Including InlineBoxKind in PositionedInlineBox Replace break_on_box with InlineBoxKind::CustomOutOfFlow Signed-off-by: Nico Burns <nico@nicoburns.com> Remove is_infinite check in alignment Signed-off-by: Nico Burns <nico@nicoburns.com> Implement yielding when height is exceeded Signed-off-by: Nico Burns <nico@nicoburns.com> Update content_widths_rtl test snapshot Signed-off-by: Nico Burns <nico@nicoburns.com> Fixup swash reference Fixup new tests Fixup vello_cpu_render example Signed-off-by: Nico Burns <nico@nicoburns.com>

Signed-off-by: Nico Burns <nico@nicoburns.com>

nicoburns mentioned this pull request Sep 26, 2025

Implements CSS Floats (float and clear properties) DioxusLabs/blitz#265

Merged

nicoburns force-pushed the floats branch 6 times, most recently from dd3a1d6 to 62d4418 Compare September 30, 2025 14:25

nicoburns force-pushed the floats branch 2 times, most recently from e7a038b to 257754b Compare October 7, 2025 13:05

nicoburns force-pushed the floats branch 11 times, most recently from f6e5cef to e55c5cb Compare November 3, 2025 14:12

nicoburns force-pushed the floats branch 5 times, most recently from 486eb53 to d8566d3 Compare November 13, 2025 12:52

nicoburns changed the title ~~WIP: Enable floats and other advanced layouts~~ Enable floats and other advanced layouts Nov 13, 2025

nicoburns marked this pull request as ready for review November 13, 2025 16:03

nicoburns requested a review from taj-p November 13, 2025 17:29

taj-p reviewed Nov 21, 2025

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Comment thread examples/swash_render/src/main.rs

nicoburns force-pushed the floats branch from c29bc37 to 4abfcbc Compare March 27, 2026 00:20

nicoburns mentioned this pull request Mar 27, 2026

Split out parley_core crate #591

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nicoburns force-pushed the floats branch 4 times, most recently from 34f30bf to c85fecd Compare March 31, 2026 18:46

taj-p self-requested a review April 2, 2026 18:11

taj-p reviewed Apr 4, 2026

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nicoburns force-pushed the floats branch from c85fecd to 11df516 Compare April 9, 2026 14:07

nicoburns requested a review from taj-p April 9, 2026 15:43

nicoburns force-pushed the floats branch from 8500e53 to 6bce6cf Compare April 9, 2026 15:45

nicoburns added this to the 0.9 milestone Apr 9, 2026

taj-p reviewed Apr 10, 2026

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taj-p approved these changes Apr 17, 2026

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Comment thread parley/src/layout/layout.rs Outdated

nicoburns added 14 commits April 17, 2026 22:24

Add float layout test

f05e646

Signed-off-by: Nico Burns <nico@nicoburns.com>

Add circle layout test

4c8d32f

Add waterfall and zigzag tests

ce086ea

Improve documentation of computed width fields

f369530

Signed-off-by: Nico Burns <nico@nicoburns.com>

Apply suggestions

89d82aa

Signed-off-by: Nico Burns <nico@nicoburns.com>

Skip out of flow boxes when doing fixed-character layout

d3c564d

Signed-off-by: Nico Burns <nico@nicoburns.com>

Implement Debug and Clone for new types

fc8eb64

Signed-off-by: Nico Burns <nico@nicoburns.com>

Removed Added by Nico comment

0b000df

Signed-off-by: Nico Burns <nico@nicoburns.com>

Update documentation for Layout::align

a61d6cd

Signed-off-by: Nico Burns <nico@nicoburns.com>

Fixup: OutOfFlow boxes affecting max-content width

35f294f

More review feedback fixes

2c46249

Add tolerance to assert to account for floating point innacuracy

f99534d

Signed-off-by: Nico Burns <nico@nicoburns.com>

Fixup doc comment

2972f69

Signed-off-by: Nico Burns <nico@nicoburns.com>

DJMcNab mentioned this pull request May 1, 2026

Restore the ability to use a different alignment width than breaking width #618

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Related PRs

Introduction

Parley's existing architecture

Changes Made

Per-line bounding box max_advance

Automatically determined (per-line) alignment width

Added max_line_height

Customisable per-line x- and y-offset

Added InlineBoxKind enum

Tasks

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nicoburns commented Sep 26, 2025 •

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Per-line bounding box `max_advance`

Added `max_line_height`

Added `InlineBoxKind` enum