Description

Re-land attempt for #30991.

From the previous version:

The caret on Android has since drifted from canonical carets on Android (comparing against stock Google apps, and blank Android textfields). Flutter's carets are significantly shorter and are not centered. This problem becomes worse when mixing multiple scripts together such as english+chinese.

This PR changes the non-iOS caret (iOS caret has special handling) to instead be the full height of the glyph at the current caret position. This causes the caret to always fully cover the glyph as well as look much closer to the stock Android behavior.

To achieve this, we add getFullHeightForCaret() to TextPainter, which returns the height the caret should be if it were to fully cover the height of the glyph. To prevent multiple calculations of the boxes around the glyphs, the metrics are cached and only recalculated if the TextPosition and caret proto passed in differ.

This fixes #23934 (as well as numerous duplicates of it)

Since this changes the caret behavior, it will likely break some golden/scuba tests, although this should be a strict improvement in fidelity compared to the current behavior. I will still label it breaking change, although there are no API breakages.

This version now adds the additional length to the bottom of the previously too-short caret, effectively centering the caret along the glyph. In previous versions, the additional height was added on either side, which made the caret skew too high.

This also removes the hardcoded 2px vertical offset from the Android caret, as it was an unscalable (as in, transformation scaling), especially for small font sizes where the 2px was a significant portion of the total height.

Checklist

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• All existing and new tests are passing.
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Breaking Change

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• Yes, this is a breaking change (Please read Handling breaking changes). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.