Fix TextField issues in RTL - #160516

…191223)

Fixes flutter#139201

## The bug

`TextPainter.computeLineMetrics` translates the paragraph's line metrics
by `paintOffset`, which is only non-zero when the text is not left
aligned. That translation built a brand new list on every call:

```dart
final List<ui.LineMetrics> rawMetrics = layout.lineMetrics;
return offset == Offset.zero
    ? rawMetrics
    : rawMetrics.map((ui.LineMetrics metrics) => _shiftLineMetrics(metrics, offset)).toList(growable: false);
```

`VerticalCaretMovementRun.isValid` uses the identity of that list to
detect text layout changes:

```dart
final List<ui.LineMetrics> newLineMetrics = _editable._textPainter.computeLineMetrics();
// Use the implementation detail of the computeLineMetrics method to figure
// out if the current text layout has been invalidated.
if (!identical(newLineMetrics, _lineMetrics)) {
  _isValid = false;
}
```

So whenever the paint offset is non-zero, a vertical caret run was
invalid the instant it was created. This matches @LongCatIsLooong's
diagnosis in
flutter#139201 (comment).

The user visible symptom is a **single line** `TextField` /
`CupertinoTextField` with `TextAlign.center` or `TextAlign.right`: a
single line field lays its text out with an unbounded width
(`RenderEditable._adjustConstraints` passes `double.infinity` as
`maxWidth` when `!_isMultiline`), so `contentWidth != paragraph.width`
and the paint offset is non-zero. Pressing the up or down arrow key then
trips `assert(isValid)` in `VerticalCaretMovementRun.moveNext` /
`movePrevious` in debug mode, and silently leaves the caret where it is
in release mode.

## The fix

Cache the translated list on `_TextPainterLayoutCacheWithOffset`, keyed
on the paint offset it was computed with (`paintOffset` is derived from
the mutable `contentWidth`, so it can change without a relayout).
`computeLineMetrics` then keeps returning the same list instance for as
long as the text layout and the paint offset are unchanged, which
restores the identity contract `VerticalCaretMovementRun` depends on. It
also stops allocating a fresh list of `LineMetrics` on every call for
every non left aligned paragraph.

## Relation to flutter#160516

flutter#160516 fixed this issue by
relaxing `isValid` to `listEquals` instead. That works, but it compares
every line's metrics on every arrow key press, it keeps allocating the
intermediate list, and it makes a run stay valid across a genuine layout
change that happens to produce equal metrics (the run's `_positionCache`
would then be stale). This PR fixes the identity contract that was
broken instead, and leaves `isValid` alone. That PR was closed for
reasons unrelated to this issue (the RTL arrow direction work for
flutter#78660).

## Tests

- `TextPainter line metrics are cached when the paint offset is not
zero` — asserts `computeLineMetrics()` returns an identical list across
calls, for every `TextAlign`.
- `TextPainter line metrics are shifted by the paint offset` — guards
the translation itself, which now happens in a new place.
- `run in a single line field that is not left aligned` in
`editable_text_shortcuts_test.dart` — the end to end regression test; it
throws the `'isValid': is not true` assertion without this patch.

All three fail on master and pass with this change. `test/painting`,
`test/rendering/editable_test.dart`,
`test/rendering/paragraph_test.dart`,
`test/widgets/editable_text_test.dart`,
`test/widgets/editable_text_shortcuts_test.dart`,
`test/widgets/text_test.dart`,
`test/widgets/selectable_region_test.dart`,
`test/material/text_field_test.dart`,
`test/material/text_form_field_test.dart`,
`test/material/selectable_text_test.dart` and
`test/cupertino/text_field_test.dart` are green.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [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 documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making.
- [x] All existing and new tests are passing.

[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/