[a11y] Re-land Make sure RenderFractionalTranslation updates its semantics after the translation field is set

[clocksmith]

Description

Re-land #48985

This PR ensures that RenderFractionalTranslation calls markNeedsSemanticsUpdate after its translation is set/changed, so that semantics are properly updated (for instance if a FractionalTranslation is animating).

Related Issues

closes #20449

Tests

I added the following tests:

• added test in proxy_box_test.dart to make sure that markNeedsSemanticsUpdate is called when changing the translation value.
• added test in basic_test.dart to make sure that the semantics bounds get updated from a FractionalTranslation translation change

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Breaking Change

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[jonahwilliams]

LGTM

[flar]

This change had a dramatic impact on the frame build times as can be seen in the flutter_gallery__transition_perf_with_semantics/delta_average_frame_build_time_millis benchmark on the flutter dashboard. The benchmark went from 2.4 to 7.4 milliseconds - or nearly triple the time.

[clocksmith]

@flar

This change fixed a critical a11y bug. Do you have any recommendations on how to preserve this behavior while remaining performant?

[flar]

It looks like this change basically just enables some aspect of the semantics code for the ProxyBox so the performance hit is likely due to the performance of the semantics code.

I don't know enough about the semantics code to make an off-hand suggestion as to how to reduce its impact. Someone more familiar with semantics should look at why the performance hit was so dramatic.

[jonahwilliams]

Unfortunately the bug this change fixed was fairly severe, so the benchmark was baselined under incorrect conditions. Similar to a rendering benchmark that didn't do any rendering, the semantics benchmark didn't do any semantics updating :(

[jonahwilliams]

That said, it is generally known that our semantics updates are computationally intensive.

[flar]

Unfortunately the bug this change fixed was fairly severe, so the benchmark was baselined under incorrect conditions. Similar to a rendering benchmark that didn't do any rendering, the semantics benchmark didn't do any semantics updating :(

Well that explains the change. The issue should probably be closed then and the benchmark re-baselined?