> Flutter apps can run much smoother precisely because they reimplement everything rather than dealing with the legacies of HTML and CSS

Maybe in some cases, but I kind of doubt this statement in general. I just tried a Flutter demo from their official site and text selection doesn't even _work_ correctly.

https://flutter.github.io/samples/web/simplistic_editor/

I'll copy-paste a few lines of the example sentence, double click on one of the middle lines to start selecting by word (which it doesn't seem to even do), and then highlighting starts on the top line instead of the line I selected.

In general the flutter apps always feel janky and second-class to the platform they're on, because they never fully implement the exact behavior of each platform they run on.