Rebuild TextBlock text runs only when the content changes

[Gillibald]

What does the pull request do?

Rebuilds a TextBlock's text runs only when the content they are built from changes, instead of on every measure pass. Embedded controls are re-measured in place, against the current constraint, rather than riding along on that rebuild: Inline.BuildTextRun loses its blockSize parameter, which only InlineUIContainer ever used, and the measuring moves to a new Inline.MeasureEmbeddedControls(Size). EmbeddedControlRun already reports its child's DesiredSize live, so the existing runs stay valid across a constraint change.

Also folds in a one-line fix for TextBlock.LineSpacing, which invalidated nothing when it changed.

What is the current behavior?

_textRuns is built from Inlines, but OnMeasureInvalidated discards it on any measure invalidation, while Inlines still holds the content. Between that point and the next measure pass the control disagrees with itself about what it contains, and CreateTextLayout reads a null _textRuns as "no inlines":

if (_textRuns != null)
    textSource = new InlinesTextSource(_textRuns);
else
    textSource = new SimpleTextSource(text ?? "", defaultProperties);

So the textSource itself is built on the wrong condition. The empty result is then shaped and stored in the TextRunCache under the text source index, so every later layout reuses it and the control never recovers on its own.

_textLayout has the same problem from the other side. InvalidateMeasure only raises OnMeasureInvalidated while the measure is still valid, and that callback is the only thing clearing the layout, so anything happening after the first invalidation leaves it in place: a second content change, or an embedded control resizing. MeasureOverride keeps that layout whenever the constraint has not moved, and a line snapshots its metrics when it is formatted, so the block measures and renders the superseded content.

What is the updated/expected behavior with this PR?

A TextBlock shapes and renders its Inlines no matter when the layout is created, and a TextRunCache entry can no longer be built from content the control does not have.

The layout is dropped by the content change itself rather than by the measure-invalidation callback, and by a measure pass that finds an embedded control has resized, so neither can leave a superseded layout in place.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

None. Inline.BuildTextRun changes signature and Inline.MeasureEmbeddedControls is new, but both are internal, and Inline cannot be derived from outside the assembly because BuildTextRun is an internal abstract member.

Obsoletions / Deprecations

None.

Fixed issues

Fixes #21902

Supersedes #21904

🤖 Generated with Claude Code

[Copilot]

🟡 Changes recommended

Repeated content invalidation before measurement can retain and reuse a stale TextLayout.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Updates TextBlock to preserve text runs across layout-only invalidations while rebuilding them when content changes.

Changes:

• Builds inline runs on demand to prevent stale text-source caching.
• Remeasures embedded controls independently of run construction.
• Adds LineSpacing invalidation and regression tests.

File summaries

File	Description
src/Avalonia.Controls/TextBlock.cs	Refactors run creation, embedded measurement, and invalidation.
src/Avalonia.Controls/SelectableTextBlock.cs	Ensures inline runs exist before layout creation.
src/Avalonia.Controls/Documents/Inline.cs	Separates run creation from embedded measurement.
src/Avalonia.Controls/Documents/InlineUIContainer.cs	Remeasures controls without rebuilding runs.
src/Avalonia.Controls/Documents/Span.cs	Propagates the separated operations.
src/Avalonia.Controls/Documents/Run.cs	Updates the run-building signature.
src/Avalonia.Controls/Documents/LineBreak.cs	Updates the run-building signature.
tests/Avalonia.Controls.UnitTests/TextBlockTests.cs	Tests caching, embedded controls, and line spacing.
tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs	Tests out-of-measure inline layout creation.
tests/Avalonia.Controls.UnitTests/InlineTests.cs	Updates calls to the new signature.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 1
• Review effort level: Balanced

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[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069441-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069453-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[Copilot]

🔵 Needs a closer look

An embedded child can be remeasured while an already-created text layout retains stale line metrics.

Review details

Suppressed comments (1)

src/Avalonia.Controls/TextBlock.cs:820

• If TextLayout is read after the block is invalidated, then an embedded child invalidates its measure before the queued measure runs, the block is already measure-invalid so OnMeasureInvalidated is not raised again. This line remeasures the child, but line 823 reuses the layout created with the child's previous size; its cached line width, height, and wrapping therefore remain stale. Dispose the layout after remeasuring so the getter rebuilds line metrics from the current DesiredSize.

            MeasureEmbeddedControls(deflatedSize);

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

The cache and layout lifetimes are consistently separated, with focused regression coverage for the identified failure modes.

Review details

• Files reviewed: 10/10 changed files
• Comments generated: 0 new
• Review effort level: Balanced

[avaloniaui-bot]

You can test this PR using the following package version. 12.2.999-cibuild0069457-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]