Add CupertinoSliverNavigationBar large title magnification on over scroll

[ivirtex]

Per #62298, adds magnification for CupertinoSliverNavigationBar large title on over scroll.
This is a continuation of my previous PR #103299, but I addressed comments and added more tests.

I decided to make use of TextPainter to calculate the width of the text and set the maximum scale appropriately,
I don't know if that's good enough because clipping will occur when the child widget is different from Text.

Also, when testing how native iOS handles this edge case, I noticed that padding applied to the large text is wrong (line 830).
This causes text to stick to the side of the screen before truncating (can be seen on the second gif).
To match the native iOS look, padding should also be applied to the end property of EdgeInsetsDirectional.only:

Padding(
  padding: const EdgeInsetsDirectional.only(
    start: _kNavBarEdgePadding,
    end: _kNavBarEdgePadding,
    bottom: 8.0, // Bottom has a different padding.
),

Fixes #62298

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[LongCatIsLooong]

As you mentioned in the pull request description, all bets are off if largeTitle.child is not a Text widget, so I think a different method for measuring the size of the child widget is needed, or alternatively you could change the constraints to prevent child from overflowing its container.

[ivirtex]

After some tests, I thought that the best way to achieve magnification effect of the large title is to extend the SingleChildRenderObjectWidget and apply the transformation just before painting.
This allows us to calculate the magnification scale on the first pass, avoiding any dry-runs.
Everything seems to work fine, magnification is correctly applied on the LTR and RTL text directions and no widget get clipped at the edge.

@LongCatIsLooong what do you think about this approach?

[LongCatIsLooong]

based on your current implementation (calculating the scale based on size.width), I think scale is probably always going to be 1.0, because the child widget is going to take as much space as possible.

[ivirtex]

based on your current implementation (calculating the scale based on size.width), I think scale is probably always going to be 1.0, because the child widget is going to take as much space as possible.

That's why I wrapped child in the Align widget, so it takes only the space it needs. I am not sure if that's the best way to handle this, but it works.

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[LongCatIsLooong]

Sorry for the delay. Some of the failing tests seem to be testing the right thing:

• the "Media padding is applied to CupertinoSliverNavigationBar" is failing because _scale is greater than 1.0 when the scroll view isn't even scrolled. Maybe we should replace the child.size.height in 1.0 + (constraints.maxHeight - child.size.height) / child.size.height * 0.03 with something else. From the looks of it, it should be _kNavBarLargeTitleHeightExtension - 8.0?
• It seems safe to just update the value from "44 - 8" to "44" from the Small title can be overridden test. The large title should have a height of 0 at that point.

[ivirtex]

• the "Media padding is applied to CupertinoSliverNavigationBar" is failing because _scale is greater than 1.0 when the scroll view isn't even scrolled. Maybe we should replace the child.size.height in 1.0 + (constraints.maxHeight - child.size.height) / child.size.height * 0.03 with something else. From the looks of it, it should be _kNavBarLargeTitleHeightExtension - 8.0?
• It seems safe to just update the value from "44 - 8" to "44" from the Small title can be overridden test. The large title should have a height of 0 at that point.

Right, I just updated the code and everything seems to be passing now.
I will make some side-to-side comparison tomorrow to check whether our 0.03 coefficient needs some tweaking.

[ivirtex]

0.03 seems to be pretty good for me.

BTW, could anyone review flutter/platform_tests#12?

[LongCatIsLooong]

LGTM, thanks for taking the time to implement this and answer my questions!

[LongCatIsLooong]

nit: make the 8 a private constant in the file?

[ivirtex]

Done!

[LongCatIsLooong]

@Hixie @MitchellGoodwin could you take another look at this PR?

[MitchellGoodwin]

LGTM

Thank you for putting this together, it will be a great addition. For resolving the conflicts, can you rebase instead of merging? It may unstick the google testing.

[ivirtex]

Whoops, I accidentally merged instead of rebasing, but all checks passed nevertheless.