[whatwg] Undoscopes inside an editable region should ignored

[Ryosuke Niwa]

Hi,

Allowing authors to define an undoscope inside an editing host appears to be
troublesome because user editing actions can modify the subtree of the host
in very complex ways, and it's hard to understand which node may be mutated
as a result of some editing actions or execCommand.

Given that, I propose to ignore undoscope content attributes or assignments
to undoScope IDL attribute if the content attribute is added to or the IDL
attribute is set to true for an editable element.

Furthermore, I propose to continue to ignore undoscope content attribute and
keep undoScope IDL attribute false when the element becomes non-editable in
the future (e.g. by removing contenteditable content attribute of its
ancestor).  Such a quirk is needed to avoid creating and destroying undoManager
as a result of style recalculation in Mozilla and WebKit because both
engines support -webkit-user-modify and -moz-user-modify to toggle
editability.

Best,
Ryosuke Niwa
Software Engineer
Google Inc.