Fix DOM-based ParagraphRuler.hitTest()

[photon3108]

Description

If you tap the nested TextSpan, it doesn't received the tap event by TapGestureRecognizer. That's because DOM-based implementation of ParagraphRuler.hitTest() doesn't pick the inner-most TextSpan up. I fixed it by arranging the search path with breadth-first traversal. There are more explanations in the comments of test-case.

Issues

Fixed: flutter/flutter#46975
Related: flutter/flutter#33523

Tests

I added a test-case for general implementation and that covers both cases of DOM-based and Canvas-based implementation.

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[photon3108]

@googlebot I signed it!

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CLAs look good, thanks!

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[mdebbar]

Thanks for fixing this bug and writing great tests and explanations!

[photon3108]

@jason-simmons Hi Jason. You were assigned to the default reviewer by Github. Are there anything I can improve this PR?

[mdebbar]

@photon3108 I don't think @jason-simmons will be reviewing this PR as it's a web-only change. @ferhatb and my approvals should be good enough. Please wait until all the checkmarks become green then merge the PR.

Thanks again for helping us improve Flutter for web!

[photon3108]

@mdebbar Github doesn't authorize me to merge this issue. All are green but there is no merge button. I thought you were waiting for @jason-simmons' review. Could you help me merge it? Thanks.

[jlubeck]

This is still broken on Flutter v1.17.0-3.4.pre

[jlubeck]

On further investigating, looks like it breaks when the TextAlign used is something other than left.
If you actually tap on the area where the text WOULD be located if left was used, it detects it there. So in my specific case, I need to tap on an empty area next to the centered paragraph to have it detect it.

@photon3108 I'm tagging you since you are the contributor.