[Selection] Reimplement SelectableText with SelectionArea

[chunhtai]

SelectableText is currently a read only EditableText which has several issues, such as the selection is bounded in a single selectable text, and it may be costly in terms of memory usage.

The idea is reimplement SelectableText with SelectionArea + text, but we should make sure they are feature on par before we do that.

several issue with SelectionArea:

#111370
#111021 （fixed）
#110594 (won't fix)
#104703 (fixed)
#104603 (fixed)
#104594
#126652 (fixed)

[exaby73]

Hey @chunhtai. Would this require documentation?

[goderbauer]

Instead of deprecating, maybe we should reimplement it with the new selection stack. It can still be useful for apps that only want to make certain pieces selectable.

[exaby73]

Wouldn't that make it redundant? If you do want to make certain pieces selectable, you can wrap those pieces in a SelectionArea, can't you?

[TahaTesser]

Hi @goderbauer!

Is there a reason that selectable_text_test.dart is under widgets while selectable_text.dart is under material?

https://github.com/flutter/flutter/blob/master/packages/flutter/lib/src/material/selectable_text.dart
https://github.com/flutter/flutter/blob/master/packages/flutter/test/widgets/selectable_text_test.dart

[goderbauer]

Is there a reason that selectable_text_test.dart is under widgets while selectable_text.dart is under material?

Looks like an oversight. The test should be under the same directory as the implementation.