Make input/textarea.selection{Direction,Start,End} not throw

[zcorpan]

Return null instead of throwing InvalidStateError for
input.selectionDirection, selectionStart, and selectionEnd, when
they do not apply.

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28225.

[zcorpan]

cc @mounirlamouri @bzbarsky

[mounirlamouri]

Sounds like a good change if it doesn't break websites assumptions (ie. have a catch, thus assume non-null).

[zcorpan]

I tried looking for things that could regress but didn't find anything:

• Not throwing was the behavior before these attributes were implemented.

• https://developer.microsoft.com/en-us/microsoft-edge/platform/catalog/ suggests Edge doesn't have selectionDirection for textarea (but does have it for input).

• httparchive gave 0 results for:

  SELECT page, url FROM (
  SELECT
    page,
    url,
    JSON_EXTRACT(payload, '$._body') AS hasBody,
    JSON_EXTRACT(payload, '$.response.content.text') AS content
  FROM [httparchive:har.chrome_feb_1_2016_requests]
  )
  WHERE hasBody = 'true'
  AND REGEXP_MATCH(content, "\bselection(Direction|Start|End)\b")
  AND REGEXP_MATCH(content, "\bcatch\b")
  

• Searching in github for a few minutes I could not find anything that would regress.

• Searching for the more generic "JSON.stringify" AND "catch" (see https://stackoverflow.com/questions/23073216/why-cant-you-stringify-a-jquery-object ) turns up lots of hits but is difficult to analyse; although there's stuff like safeJsonStringify which tries JSON.stringify and falls back to a custom function.

[bzbarsky]

Seems ok to me; please update web platform tests as needed to make sure implementors get this on their radar or something..

[smaug----]

ok to me

[zcorpan]

Tests at web-platform-tests/wpt#2825

[zcorpan]

Bugs:
https://bugs.webkit.org/show_bug.cgi?id=156500
https://bugs.chromium.org/p/chromium/issues/detail?id=622679
https://bugzilla.mozilla.org/show_bug.cgi?id=1281769
(I tried reporting it twice for Edge but got "unexpected error". cc @gregwhitworth)