fix focus traversal after pages popped

[chunhtai]

Description

There are two mechanism that traverse the focus in ModalRoute when a page pop, and they are not in sync and conflict with each other

flutter/packages/flutter/lib/src/widgets/routes.dart

Line 674 in 22fc215

if (widget.route.isCurrent) {

and

flutter/packages/flutter/lib/src/widgets/routes.dart

Line 704 in 22fc215

AnimatedBuilder(

The first one request focus regardless the state of animation controller, while the second one enable focus based on the state of animation controller. This makes focus node traverse to a weird place when pops more than two pages.

for example, we have three pages in routes history
page1
page2
page3 <--- current focus

we pop page2 and page3 at the same time, as soon as the animation controller is reverse we trigger

flutter/packages/flutter/lib/src/widgets/routes.dart

Line 674 in 22fc215

if (widget.route.isCurrent) {

page3 will trigger first and then page2, the bring the focus to page2 before the animation has even starting the build the first frame.

flutter/packages/flutter/lib/src/widgets/routes.dart

Line 704 in 22fc215

AnimatedBuilder(

will try to bring the focus from page3 to page2, and then page2 to page1. However, the focus is already on page2, and there are series of FocusNode marked dirty with nextnode = null which brings the focus to be on root focus node and cause this error.

Related Issues

fixes #48903

Tests

I added the following tests:

"focus traverse correct when pop mutiple page simultaneously"

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[gspencergoog]

Thanks!

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