setRangeText should probably set the dirty value flag

[bzbarsky]

Consider this testcase:

<textarea>abcde</textarea>
<script>
  var t = document.querySelector("textarea");
  t.selectionStart = 1;
  t.selectionEnd = 2;
  t.setRangeText("xyz");
  t.defaultValue = "Why am I here?";
  alert(t.value);
</script>

Per spec this should alert (and show in the textarea) "Why am I here?" because https://html.spec.whatwg.org/multipage/forms.html#dom-textarea/input-setrangetext never sets the dirty value flag. That's not what what UAs do. They alert "axyzcde" (or at least Chrome, Safari, and Firefox do). As far as I can tell this is a spec bug; setRangeText should set the flag.

@cdumez @annevk @domenic @travisleithead

[bzbarsky]

I should note that Gecko alerts "Why am I here?" if <input> is used instead of <textarea>. I consider that a bug that I will fix to make the two match.

[bzbarsky]

I have changed Gecko behavior for <input> to match other UAs and <textarea>.

Once this is sorted out, there is a test at the end of https://hg.mozilla.org/mozilla-central/file/7b19a6386225/testing/web-platform/mozilla/tests/html/semantics/forms/textfieldselection/selection-value-interactions.html that can be moved to a normal wpt test.

[rniwa]

I agree this is a spec bug. We should just make this spec text change.

[bzbarsky]

Test moved over to the "to be upstreamed" bucket: https://hg.mozilla.org/integration/mozilla-inbound/rev/7b89ee0ae4204848f8d6f8f03ea036b6eeff3814