[web_paragraph] Apply StrutStyle to line metrics and placeholders - #193774

…flutter#193733)

Part of flutter#193731. Split out of flutter#193403 (first of four independent PRs;
the others are flutter#193734 and flutter#193736, and flutter#193403 itself will be rebased
on top once these land).

### Problem
- `ChildStyleNode._height` converted `style.height ==
ui.kTextHeightNone` to `null` *before* falling back to `parent._height`,
so a child `TextStyle(height: ui.kTextHeightNone)` could never override
a non-null parent height (`ParagraphStyle(height: 3.0)` +
`pushStyle(TextStyle(height: kTextHeightNone))` still laid out the line
at 3x).
- `LineBlock` only checked `span.style.height == null`, so
`ParagraphStyle(height: ui.kTextHeightNone)` (`0.0`) multiplied the line
ascent/descent by `0.0` instead of disabling the height multiplier.

### Fix
- `ChildStyleNode._height` keeps `style.height` as-is: `style.height ??
parent._height`. `kTextHeightNone` now reaches `LineBlock`, which is the
one place that interprets it.
- `LineBlock`: `height == null || height == ui.kTextHeightNone` → raw
font ascent/descent, no scaling (same as the native path, where
`paragraph_builder.cc` sets `has_height_override = style.height !=
kTextHeightNone`).
- `LineBlock` uses `StyleManager.defaultFontSize` instead of a hardcoded
`14.0`.
- `WebStrutStyle` already normalizes `kTextHeightNone` to `null` in its
constructor, so struts need no change.

### Tests
- `paragraph_get_boxes_test.dart`: `layout with kTextHeightNone disables
parent height multiplier`, `layout with
ParagraphStyle(kTextHeightNone)`.
- **WebParagraph corpus (`preferWebParagraph` + `forceTestFonts` vs
`origin/master` `b7315d76579e`)**: 20,012 tests (18,765 executed, 1,247
skipped) — **503 -> 503 failures (0 fixed standalone, +2 new tests
passing, 0 regressions)**.
- *Note:* Existing corpus tests for `kTextHeightNone` also depend on
other issues (`test/ui/paragraph_builder_test.dart: kTextHeightNone
StrutStyle` is unlocked in flutter#193774 once `StrutStyle` is applied;
`test/painting/text_painter_test.dart: kTextHeightNone unsets the text
height multiplier` still fails due to Chromium's integer-rounded
`FlutterTest` font metrics at `10px` where `ascent + descent = 11 !=
10`).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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…ult to proportional (flutter#193736)

Part of flutter#193731. Split out of flutter#193403 (third of four independent PRs;
the others are flutter#193733 and flutter#193734, and flutter#193403 is stacked on top of
this PR).

### Problem
- **`TextHeightBehavior.leadingDistribution` was ignored.**
`WebParagraphStyle` never passed
`textHeightBehavior.leadingDistribution` to the root `WebTextStyle`, and
`RootStyleNode._leadingDistribution` always returned `null`, so the
value could not reach `LineBlock`. Only an explicit
`TextStyle(leadingDistribution: …)` had any effect.
- **The default leading distribution was `even`.** `LineBlock` treated
`null` the same as `TextLeadingDistribution.even` (half-leading).
SkParagraph (and therefore the native engine and CanvasKit) default to
`proportional`: `TextStyle::fHalfLeading` is `false` unless
`leadingDistribution == even` is set, and `dart:ui`'s
`TextHeightBehavior.leadingDistribution` defaults to
`TextLeadingDistribution.proportional`.
- **Negative half-leading clamped `fontBoundingBoxDescent` to `0.0`.**
`TextLine` initialized `fontBoundingBoxAscent` and
`fontBoundingBoxDescent` to `0.0` and accumulated every block with
`math.max(0.0, ...)`. With `TextLeadingDistribution.even` and a small
`height` multiplier (where `height * fontSize < rawAscent +
rawDescent`), `extraLeading / 2` is negative and
`multipliedFontBoundingBoxDescent` can become negative; `math.max(0.0,
...)` clamped it to `0.0`, inflating the line height above `height *
fontSize`.

### Fix
- `WebParagraphStyle` propagates
`textHeightBehavior?.leadingDistribution` into the root `WebTextStyle`;
`RootStyleNode._leadingDistribution` returns
`style.leadingDistribution`. Child `TextStyle`s still override it
through `ChildStyleNode` as before.
- `LineBlock`: `null` now falls through to the `proportional` branch
(ascent and descent are scaled by `height * fontSize / (ascent +
descent)`), `even` keeps the half-leading behavior.
- `TextLine.updateBoundingBox`: track `_hasLineMetrics` so the first
block on the line initializes `fontBoundingBoxAscent` and
`fontBoundingBoxDescent` directly (preserving a negative `descent`),
while subsequent blocks take `math.max`.

### Behavior change
For any paragraph with a `height` multiplier and no explicit
`leadingDistribution`, the **line height is unchanged** but the
**baseline moves**: the extra leading is now split in proportion to the
font's ascent/descent instead of 50/50. This brings WebParagraph in line
with CanvasKit/Skwasm output for the same `ParagraphStyle`; WebParagraph
is not the default renderer yet, so no app-facing change on stable.

### Tests
- New `paragraph_line_height_test.dart`:
- `Height multiplier scales ascent and descent proportionally by
default` — checks the scaled line keeps `height * fontSize`, that
ascent/descent scale by the same factor by default, that
`TextHeightBehavior(leadingDistribution: even)` adds equal leading on
both sides, and that the two differ. Fails on `master` (default was
`even` and the THB value was ignored).
- `Half-leading with a small height multiplier preserves negative
descent` — checks that `height: 0.1` with `TextLeadingDistribution.even`
keeps a negative `descent` and total height `0.1 * fontSize`.
- Existing `webparagraph/*` and `ui/paragraph_builder_test.dart` suites
pass.
- **WebParagraph corpus (`preferWebParagraph` + `forceTestFonts` vs
`origin/master` `b7315d76579e`)**: **0 fixed standalone, +2 new tests
passing, 0 regressions** (base of the flutter#193736 -> flutter#193773 -> flutter#193774 ->
flutter#193403 stack that fixes **34** corpus tests; framework `TextPainter
line-height` tests in `test/painting/text_painter_test.dart` also call
`getBoxesForSelection` with `BoxHeightStyle.tight` and are unlocked when
combined with flutter#193734).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

Co-authored-by: Mouad Debbar <[email protected]>

…boxes (flutter#193734)

Part of flutter#193731. Split out of flutter#193403 (second of four independent PRs;
the others are flutter#193733 and flutter#193736, and flutter#193403 itself will be rebased
on top once these land).

### Problem
- **Placeholder vertical position was computed too early.**
`PlaceholderBlock.calculatePlaceholderTop` set `block.advance` from the
line ascent *as it was at that moment* before folding `block.ascent`
into the line. A placeholder taller than the preceding text (or a later,
taller placeholder on the same line) grew `line.fontBoundingBoxAscent`
afterwards in `line.updateBoundingBox(block)`, leaving placeholder
`advance.top` positioned against a stale ascent.
`getBoxesForPlaceholders` returned those stale rects, so a placeholder
box could sit above the line's top (negative `top`) and disagree with
`getBoxesForRange` for the same range.
- **`BoxHeightStyle.tight` returned the height-scaled box for text.**
`getBoxesForRange` used `multipliedFontBoundingBoxAscent` /
`multipliedHeight`, i.e. the box included the extra leading added by
`TextStyle.height`. SkParagraph's `TextLine::getRectsForRange` returns
the unscaled glyph bounds for `tight` (`Rect::MakeXYWH(...,
-run.ascent(), ..., run.descent())` relative to the baseline). The same
code path was also applied to `PlaceholderBlock`, whose `advance` is
already line-relative, producing a double-shifted box.

### Fix
- `PlaceholderBlock` & `addLine`: split placeholder layout into two
passes:
- Pass 1 calls
`block.calculatePlaceholderMetrics(line.fontBoundingBoxAscent,
line.fontBoundingBoxDescent)` to compute each placeholder's
`ascent`/`descent` and folds them into the line metrics via
`line.updateBoundingBox(block)`.
- Pass 2, after `line.fontBoundingBoxAscent` is final, calls
`block.calculatePlaceholderTop(line.fontBoundingBoxAscent)` to compute
`top = lineAscent - ascent` and set `block.advance =
Rect.fromLTWH(spanShiftFromLineStart, top, span.width, span.height)`.
- `getBoxesForRange`, `BoxHeightStyle.tight`:
- `TextBlock`: `top = line.advance.top + line.fontBoundingBoxAscent -
block.rawFontBoundingBoxAscent`, `height = rawFontBoundingBoxAscent +
rawFontBoundingBoxDescent` (unscaled glyph bounds positioned at the
baseline, matching SkParagraph).
- `PlaceholderBlock`: `top = firstRect.top + line.advance.top`, `height
= span.height` — `block.advance` is already relative to the line, so no
extra ascent shift.
- Removed the unused vertical `.translate(...)` on `firstRect` for
`TextBlock`, since `firstRect` only provides `left` and `width` while
`top` and `bottom` are computed per `BoxHeightStyle`.

### Tests
- `paragraph_get_boxes_test.dart`:
- `getBoxesForRange and getBoxesForPlaceholders return matching boxes
for placeholders` — two placeholders on one line where the second is
taller; checks both APIs agree, `top >= 0`, and the box sizes equal the
placeholder sizes. On `master`: `getBoxesForPlaceholders` returns `(0,
-12, 50, 18)` for the first placeholder while `getBoxesForRange(0, 1)`
returns `(0, 14, 50, 44)`.
- `BoxHeightStyle.tight uses unscaled glyph height and respects
TextHeightBehavior` — `height: 3.0` with `even` leading; `tight` box
height equals the unscaled line height and is centered in the 60px line.
On `master` the `tight` box is 60px tall instead of 22px.
- Existing `paragraph_get_boxes_test.dart` and
`paragraph_placeholders_test.dart` pass.
- **Full WebParagraph corpus (`--full`, `preferWebParagraph` +
`forceTestFonts` vs `origin/master` `b7315d76579e`)**:
- Part 1 (`web_ui`): `16/16` `webparagraph/*` (`+198 ~5 -1`, **+2** new
tests passing) + `60/60` `test/ui/*` (`+1354 ~5 -41`).
- Part 2 (`packages/flutter`, `894/894` suites across `8/8` shards):
`+16712 ~1237 -459` vs baseline `+16710 ~1237 -461` (**+3** existing
tests fixed; **1** apparent golden diff in
`test/cupertino/text_field_test.dart: Overflow clipBehavior none golden`
where on `origin/master` two bugs cancelled out — missing
`StrutStyle(forceStrutHeight: true)` from flutter#193774 inflated the line
ascent by 83px while the old single-pass `calculatePlaceholderTop`
shifted the placeholder by `-83px` detached from its `'Hi'` text;
resolved once flutter#193774 lands).
- Jointly with flutter#193736 + flutter#193773 also unlocks
`test/painting/text_painter_test.dart: TextPainter line-height
half-leading with leading trim`.

<details>
<summary>Fixed corpus tests vs <code>origin/master</code> (3)</summary>

- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter line-height TextLeadingDistribution falls back to paragraph
style`
- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter line-height half-leading`
- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter widget span`
</details>

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
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- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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…flutter#193733)

Part of flutter#193731. Split out of flutter#193403 (first of four independent PRs;
the others are flutter#193734 and flutter#193736, and flutter#193403 itself will be rebased
on top once these land).

### Problem
- `ChildStyleNode._height` converted `style.height ==
ui.kTextHeightNone` to `null` *before* falling back to `parent._height`,
so a child `TextStyle(height: ui.kTextHeightNone)` could never override
a non-null parent height (`ParagraphStyle(height: 3.0)` +
`pushStyle(TextStyle(height: kTextHeightNone))` still laid out the line
at 3x).
- `LineBlock` only checked `span.style.height == null`, so
`ParagraphStyle(height: ui.kTextHeightNone)` (`0.0`) multiplied the line
ascent/descent by `0.0` instead of disabling the height multiplier.

### Fix
- `ChildStyleNode._height` keeps `style.height` as-is: `style.height ??
parent._height`. `kTextHeightNone` now reaches `LineBlock`, which is the
one place that interprets it.
- `LineBlock`: `height == null || height == ui.kTextHeightNone` → raw
font ascent/descent, no scaling (same as the native path, where
`paragraph_builder.cc` sets `has_height_override = style.height !=
kTextHeightNone`).
- `LineBlock` uses `StyleManager.defaultFontSize` instead of a hardcoded
`14.0`.
- `WebStrutStyle` already normalizes `kTextHeightNone` to `null` in its
constructor, so struts need no change.

### Tests
- `paragraph_get_boxes_test.dart`: `layout with kTextHeightNone disables
parent height multiplier`, `layout with
ParagraphStyle(kTextHeightNone)`.
- **WebParagraph corpus (`preferWebParagraph` + `forceTestFonts` vs
`origin/master` `b7315d76579e`)**: 20,012 tests (18,765 executed, 1,247
skipped) — **503 -> 503 failures (0 fixed standalone, +2 new tests
passing, 0 regressions)**.
- *Note:* Existing corpus tests for `kTextHeightNone` also depend on
other issues (`test/ui/paragraph_builder_test.dart: kTextHeightNone
StrutStyle` is unlocked in flutter#193774 once `StrutStyle` is applied;
`test/painting/text_painter_test.dart: kTextHeightNone unsets the text
height multiplier` still fails due to Chromium's integer-rounded
`FlutterTest` font metrics at `10px` where `ascent + descent = 11 !=
10`).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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…ult to proportional (flutter#193736)

Part of flutter#193731. Split out of flutter#193403 (third of four independent PRs;
the others are flutter#193733 and flutter#193734, and flutter#193403 is stacked on top of
this PR).

### Problem
- **`TextHeightBehavior.leadingDistribution` was ignored.**
`WebParagraphStyle` never passed
`textHeightBehavior.leadingDistribution` to the root `WebTextStyle`, and
`RootStyleNode._leadingDistribution` always returned `null`, so the
value could not reach `LineBlock`. Only an explicit
`TextStyle(leadingDistribution: …)` had any effect.
- **The default leading distribution was `even`.** `LineBlock` treated
`null` the same as `TextLeadingDistribution.even` (half-leading).
SkParagraph (and therefore the native engine and CanvasKit) default to
`proportional`: `TextStyle::fHalfLeading` is `false` unless
`leadingDistribution == even` is set, and `dart:ui`'s
`TextHeightBehavior.leadingDistribution` defaults to
`TextLeadingDistribution.proportional`.
- **Negative half-leading clamped `fontBoundingBoxDescent` to `0.0`.**
`TextLine` initialized `fontBoundingBoxAscent` and
`fontBoundingBoxDescent` to `0.0` and accumulated every block with
`math.max(0.0, ...)`. With `TextLeadingDistribution.even` and a small
`height` multiplier (where `height * fontSize < rawAscent +
rawDescent`), `extraLeading / 2` is negative and
`multipliedFontBoundingBoxDescent` can become negative; `math.max(0.0,
...)` clamped it to `0.0`, inflating the line height above `height *
fontSize`.

### Fix
- `WebParagraphStyle` propagates
`textHeightBehavior?.leadingDistribution` into the root `WebTextStyle`;
`RootStyleNode._leadingDistribution` returns
`style.leadingDistribution`. Child `TextStyle`s still override it
through `ChildStyleNode` as before.
- `LineBlock`: `null` now falls through to the `proportional` branch
(ascent and descent are scaled by `height * fontSize / (ascent +
descent)`), `even` keeps the half-leading behavior.
- `TextLine.updateBoundingBox`: track `_hasLineMetrics` so the first
block on the line initializes `fontBoundingBoxAscent` and
`fontBoundingBoxDescent` directly (preserving a negative `descent`),
while subsequent blocks take `math.max`.

### Behavior change
For any paragraph with a `height` multiplier and no explicit
`leadingDistribution`, the **line height is unchanged** but the
**baseline moves**: the extra leading is now split in proportion to the
font's ascent/descent instead of 50/50. This brings WebParagraph in line
with CanvasKit/Skwasm output for the same `ParagraphStyle`; WebParagraph
is not the default renderer yet, so no app-facing change on stable.

### Tests
- New `paragraph_line_height_test.dart`:
- `Height multiplier scales ascent and descent proportionally by
default` — checks the scaled line keeps `height * fontSize`, that
ascent/descent scale by the same factor by default, that
`TextHeightBehavior(leadingDistribution: even)` adds equal leading on
both sides, and that the two differ. Fails on `master` (default was
`even` and the THB value was ignored).
- `Half-leading with a small height multiplier preserves negative
descent` — checks that `height: 0.1` with `TextLeadingDistribution.even`
keeps a negative `descent` and total height `0.1 * fontSize`.
- Existing `webparagraph/*` and `ui/paragraph_builder_test.dart` suites
pass.
- **WebParagraph corpus (`preferWebParagraph` + `forceTestFonts` vs
`origin/master` `b7315d76579e`)**: **0 fixed standalone, +2 new tests
passing, 0 regressions** (base of the flutter#193736 -> flutter#193773 -> flutter#193774 ->
flutter#193403 stack that fixes **34** corpus tests; framework `TextPainter
line-height` tests in `test/painting/text_painter_test.dart` also call
`getBoxesForSelection` with `BoxHeightStyle.tight` and are unlocked when
combined with flutter#193734).

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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---------

Co-authored-by: Mouad Debbar <[email protected]>

…boxes (flutter#193734)

Part of flutter#193731. Split out of flutter#193403 (second of four independent PRs;
the others are flutter#193733 and flutter#193736, and flutter#193403 itself will be rebased
on top once these land).

### Problem
- **Placeholder vertical position was computed too early.**
`PlaceholderBlock.calculatePlaceholderTop` set `block.advance` from the
line ascent *as it was at that moment* before folding `block.ascent`
into the line. A placeholder taller than the preceding text (or a later,
taller placeholder on the same line) grew `line.fontBoundingBoxAscent`
afterwards in `line.updateBoundingBox(block)`, leaving placeholder
`advance.top` positioned against a stale ascent.
`getBoxesForPlaceholders` returned those stale rects, so a placeholder
box could sit above the line's top (negative `top`) and disagree with
`getBoxesForRange` for the same range.
- **`BoxHeightStyle.tight` returned the height-scaled box for text.**
`getBoxesForRange` used `multipliedFontBoundingBoxAscent` /
`multipliedHeight`, i.e. the box included the extra leading added by
`TextStyle.height`. SkParagraph's `TextLine::getRectsForRange` returns
the unscaled glyph bounds for `tight` (`Rect::MakeXYWH(...,
-run.ascent(), ..., run.descent())` relative to the baseline). The same
code path was also applied to `PlaceholderBlock`, whose `advance` is
already line-relative, producing a double-shifted box.

### Fix
- `PlaceholderBlock` & `addLine`: split placeholder layout into two
passes:
- Pass 1 calls
`block.calculatePlaceholderMetrics(line.fontBoundingBoxAscent,
line.fontBoundingBoxDescent)` to compute each placeholder's
`ascent`/`descent` and folds them into the line metrics via
`line.updateBoundingBox(block)`.
- Pass 2, after `line.fontBoundingBoxAscent` is final, calls
`block.calculatePlaceholderTop(line.fontBoundingBoxAscent)` to compute
`top = lineAscent - ascent` and set `block.advance =
Rect.fromLTWH(spanShiftFromLineStart, top, span.width, span.height)`.
- `getBoxesForRange`, `BoxHeightStyle.tight`:
- `TextBlock`: `top = line.advance.top + line.fontBoundingBoxAscent -
block.rawFontBoundingBoxAscent`, `height = rawFontBoundingBoxAscent +
rawFontBoundingBoxDescent` (unscaled glyph bounds positioned at the
baseline, matching SkParagraph).
- `PlaceholderBlock`: `top = firstRect.top + line.advance.top`, `height
= span.height` — `block.advance` is already relative to the line, so no
extra ascent shift.
- Removed the unused vertical `.translate(...)` on `firstRect` for
`TextBlock`, since `firstRect` only provides `left` and `width` while
`top` and `bottom` are computed per `BoxHeightStyle`.

### Tests
- `paragraph_get_boxes_test.dart`:
- `getBoxesForRange and getBoxesForPlaceholders return matching boxes
for placeholders` — two placeholders on one line where the second is
taller; checks both APIs agree, `top >= 0`, and the box sizes equal the
placeholder sizes. On `master`: `getBoxesForPlaceholders` returns `(0,
-12, 50, 18)` for the first placeholder while `getBoxesForRange(0, 1)`
returns `(0, 14, 50, 44)`.
- `BoxHeightStyle.tight uses unscaled glyph height and respects
TextHeightBehavior` — `height: 3.0` with `even` leading; `tight` box
height equals the unscaled line height and is centered in the 60px line.
On `master` the `tight` box is 60px tall instead of 22px.
- Existing `paragraph_get_boxes_test.dart` and
`paragraph_placeholders_test.dart` pass.
- **Full WebParagraph corpus (`--full`, `preferWebParagraph` +
`forceTestFonts` vs `origin/master` `b7315d76579e`)**:
- Part 1 (`web_ui`): `16/16` `webparagraph/*` (`+198 ~5 -1`, **+2** new
tests passing) + `60/60` `test/ui/*` (`+1354 ~5 -41`).
- Part 2 (`packages/flutter`, `894/894` suites across `8/8` shards):
`+16712 ~1237 -459` vs baseline `+16710 ~1237 -461` (**+3** existing
tests fixed; **1** apparent golden diff in
`test/cupertino/text_field_test.dart: Overflow clipBehavior none golden`
where on `origin/master` two bugs cancelled out — missing
`StrutStyle(forceStrutHeight: true)` from flutter#193774 inflated the line
ascent by 83px while the old single-pass `calculatePlaceholderTop`
shifted the placeholder by `-83px` detached from its `'Hi'` text;
resolved once flutter#193774 lands).
- Jointly with flutter#193736 + flutter#193773 also unlocks
`test/painting/text_painter_test.dart: TextPainter line-height
half-leading with leading trim`.

<details>
<summary>Fixed corpus tests vs <code>origin/master</code> (3)</summary>

- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter line-height TextLeadingDistribution falls back to paragraph
style`
- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter line-height half-leading`
- `packages/flutter` :: `test/painting/text_painter_test.dart:
TextPainter widget span`
</details>

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant in-code documentation (doc comments with
`///`).
- [x] If this PR introduces a new feature or capability, I created and
linked a website documentation issue or PR in [flutter/website] (or
verified none is needed).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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