Fix Rect::ExpandToMinTransformedSize to return the input rectangle when no expansion is needed, and remove 1-pixel roundrect to rect simplification

[b-luk]

Fixes #189807

See the issue for more context.

Adds a unit test that fails without this change, and passes with this change.

Verified the demo app in the issue renders properly with this change.

From the issue:

Root cause and fix

Rect::ExpandToMinTransformedSize always constructs a new expanded rectangle from the input Rect from its center point and half-extents.

If the input rectangle is already large enough and does not need expansion, this should return an identical rectangle. The returned rectangle is equality-checked against the input in Canvas::DrawRoundRect to determine whether expansion happened. That code assumes that if the output does not equal the input, pixel-expansion happened, which means that the rounded rect had sub-pixel dimensions and can be safely drawn as a non-rounded rectangle.

The issue is that for certain cases, particularly with non-integer sizes, ExpandToMinTransformedSize returns a different rectangle even if no expansion happened. This is due to small floating point imprecisions when constructing the new rectangle using the input rectangle's center and half-extents.

To fix this, ExpandToMinTransformedSize should not construct a new rectangle unless expansion is actually needed.

Removal of 1-pixel roundrect to rect simplification

This PR also removes the 1-pixel roundrect to rect simplification in Canvas::DrawRoundRect. As described in #189807 (comment), this optimization resulted in slightly different colors at the tips of thin roundrects.

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[b-luk]

/gemini review

[gemini-code-assist]

Code Review

This pull request updates the rectangle expansion logic in rect.h to return the original rectangle without modification if its current size already meets or exceeds the minimum local size. Additionally, a unit test has been renamed and a new regression test has been added in rect_unittests.cc to verify this behavior with fractional sizes. There are no review comments, and I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request modifies the rectangle expansion logic in Impeller's geometry library to return the unmodified rectangle when its current dimensions are already greater than or equal to the minimum required size. Additionally, a regression test has been added to verify this behavior with fractional dimensions. There are no review comments, and I have no feedback to provide.

[b-luk]

@flar PTAL. I went with your suggestion in #189807 (comment) and also removed the 1-pixel roundrect to rect simplification.

The golden results show a very subtle reduction in opacity of pixels at the end of sub-pixel-width roundrects. That is what we expect from this change: https://flutter-gold.skia.org/search?crs=github&issue=189808&patchsets=2

[flar]

LGTM