The only difference in behavior I see is moving marker icon when selecting with arrow keys, but I doubt it's better than keeping marker icon near mouse pointer.

When selecting with keyboard the pageX and pageY of event would be a 0. That's why preferred to use Range#getClientRects to find position.

Currently marker appears when you release mouse button and doesn't move if you change selection with keyboard. If we apply your patch marker will move with end of selection, but you still need mouse to click it, so people have to move mouse to new marker position. I doubt that this behavior is userfriendly.

Moving marker is more convenient when selecting long text in small srollable area. But I otherwise I'd prefer current behavior.

I think we could use Range#getClientRects to calculate marker position more correctly and to avoid the situation when non selected text overlapped with marker button.

все правильно, pageX и pageY будут равны 0.
но я сомневаюсь, что выделение отдельных фрагментов в тексте удобно делать с клавиатуры.
в любом случае, мы чуть позже рассмотрим этот pull request.