[HarfBuzz] Better handling variation selectors

[Jonathan Kew]

On 23/4/13 02:10, Behdad Esfahbod wrote:
> On 13-04-20 05:49 AM, Jonathan Kew wrote:
>> On 19/4/13 22:13, Behdad Esfahbod wrote:
>>> Konstantin has brought up a few interesting issues regarding variation
>>> selectors.  I'll summarize issues that I think can be improved:
>>>
>>>     - Right now we don't handle a sequence of multiple variation selectors in a
>>> row.  What *is* the best way to handle that?  Ideally the extra ones should
>>> become visible.  Even a box is better than silently not showing them, right?
>>
>> No - silently not showing them should be the default behavior, IMO. They're
>> default-ignorable.
>
> I disagree a bit here.  What Default_Ignorable really means is: "if you don't
> know what to do with them, don't show them".  In this case, we do know what to
> do with them, and have multiple ones on the same character is an encoding
> error, so showing something may be the better choice.

It's no more of an encoding error than a <char, VS> pair that is not a 
registered variation sequence. If we're going to make "stray" variation 
selectors visible by default, then I think we'd need to check the 
validity of all VS occurrences, so that we can consistently make the 
"strays" visible, rather than showing some of them and ignoring others.

JK