[whatwg] Request for implementer feedback: granularity definitions for Selection.modify()

[Aryeh Gregor]

I'm working on a definition of Selection.modify(), and have run into
the problem of how to define the various granularities used there.
(See <https://developer.mozilla.org/en/DOM/selection/modify> for basic
docs.)  The intent of the API is clearly that, e.g.,
selection.modify("move", "right", "character") should do something
like hitting the right arrow key, selection.modify("extend", "right",
"line") should be like Shift+Down, etc.  But this is platform-specific
in practice -- e.g., Ctrl+Left and Ctrl+Right use different
definitions of word boundaries in different programs I use.

To what degree should I try to define how these granularities work?  I
could go ahead and say that moving by characters should be done by
moving to the next grapheme cluster boundary as defined by UAX#29, for
example.  Or I could say that it should follow platform conventions
for when the user advances by a character, e.g., using the right and
left arrow keys.  Or maybe something else?  What would implementers
prefer?

In practice, in other places where this stuff is needed (like
line-breaking or :first-letter), we just leave it entirely undefined,
which doesn't seem ideal but perhaps isn't fully avoidable.