Fix Exception on Nested TabBarView disposal

[shihaohong]

Description

When a TabBarView is nested in a page of another TabBarView, there will be particular scenarios that will throw an exception.

To reproduce this issue, two conditions must be true:

1. Tap on a tab that is adjacent to a tab containing a nested TabBarView
2. The nested tab has to be in between the initial tab and the tab that was tapped on
   (ie. current index = 0, tap on tab at index 3 when tab at index 2 has nested TabBarView)

This happens when the framework tries to build _PagePosition, then dispose of it before applyViewportDimension has the chance to be called to set pixels to a non-null value based on the size of the viewport. This fix adds a flag to determine if applyViewportDimension is invoked before dispose. If it hasn't been, then dispose will set pixels to an arbitrary value before attempting to dispose of it.

First exception: This is due to the introduction of #29188, which happens because the initial swap that is necessary for warping causes the nested page that is swapped but not shown to be disposed of before applyViewportDimension to be called.

Second exception: This one is caused by the animation of the page during warp. The last frame does not build before the second setState is called to swap the children back to their original positions, causing the nested page to be built and disposed of before applyViewportDimension can be called.

TODO:

• Add an issue to track adding a test to ensure that the warp animation for TabBarView builds its last frame before setState is called to re-swap the children back to their original locations. ScrollPosition.animateTo should build last frame before resolving animation #32054
• Track the potential need for a more robust swapping mechanism that doesn't cause pages that are not shown to be built and immediately disposed of. Tab _warpToCurrentIndex causing pages to build and dispose immediately #32056

Related Issues

Fixes #18756

Tests

I added the following tests:

• A regression test to ensure that the exception does throw when a tab adjacent to nested tab is selected. It tests for the first exception case mentioned above. A separate issue will be created to track writing a test for the second exception case.

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Breaking Change

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[chunhtai]

#31978 merged

[HansMuller]

This is definitely a workaround. Hopefully we'll be able to replace it with fixes for #32054, #32056.

[chunhtai]

can you add a TODO and link to the issue and note once the issue is fix we can remove this workaround

[shihaohong]

Done!