Text selection after updating textContent of TEXTAREA

[tkent-google]

The specification says:
https://html.spec.whatwg.org/multipage/forms.html#the-textarea-element:textcontent

When the textarea element's textContent IDL attribute changes value, if the element's dirty value flag is false, then the element's raw value must be set to the value of the element's textContent IDL attribute.

This paragraph doesn't mention text selection. I confirmed Chrome, Edge, Firefox, and Safari set text selection to the end of the new value. So I propose to update the paragraph like the value setter.

https://html.spec.whatwg.org/multipage/forms.html#dom-textarea-value

and should then move the text entry cursor position to the end of the text control, unselecting any selected text and resetting the selection direction to none.

[rniwa]

I guess there's also a question as to what happens to the selection returned by getSelection(). There's some discrepancy between WebKit/Blink and Gecko/Edge in that regard.

[domenic]

Hopefully getSelection() will return nothing, if visually no text is selected?

I'm happy to work on the change here and write some web platform tests. I'll be sure to include getSelection() as well as the stuff HTML defines (selectionStart/selectionEnd/selectionDirection).

[tkent-google]

I filed w3c/selection-api#83 for the issue of getSelection() and INPUT/TEXTAREA.

[bzbarsky]

I confirmed Chrome, Edge, Firefox, and Safari set text selection to the end of the new value.

That's ... not necessarily true. See discussion in #2424