While I disagree with most of what you've said on this list, it is not an unreasonable proposal to change the default properties for some ranges of the private use blocks. I don't think that this would, in practice, really disturb any applications, because of #1 below.
I have, however, a few observations.
1. PUA properties, as is clear from Ken's excellent descriptions, are simply
defaults. With the exception of normalization, no Unicode implementation is
required to observe them. So even if this change is made, any conformant
implementation is free to simply ignore it and just assign its own properties.
This would not be a magic wand.
Understood. But I was rather thinking that at least some implementations base their character properties directly on the Unicode character database. Isn't this what ICU does? And so, if the PUA default properties are the ones in the UCD, they would automatically be used by implementations.
2. Unicode properties are not sufficient for rendering. With technologies such
as Apples, all of the other work can be done in a font. With OpenType, most but
not all can -- in particular, reordering has to be done by the application/OS.
So complex scripts that require reordering still would not be interchangeable
without private agreement.
This is why the suggestions made for storing character properties in the font are unrealistic; they require major restructuring of system software (close to rewriting the whole OS, as I wrote earlier), not just tinkering. I accept that there may be some practical limitations on PUA complex scripts, but I would like them to be a lot less than they are now.
3. Even excluding the normalization properties and other obvious inapplicable properties (such as name or age), there are some 50-odd possible character properties, many of them with multiple possible values: see
http://www.unicode.org/Public/UNIDATA/PropertyAliases.txt http://www.unicode.org/Public/UNIDATA/UCD.html#Properties http://www.unicode.org/Public/UNIDATA/PropertyValueAliases.txt
A concrete proposal would have to specify exactly which properties were
relevant, and what the values are for the proposed ranges. (Clearly an even
partition according to all the possible combinations would be completely
impractical.) If the goal is rendering, this means looking at the possible
combinations of properties that are relevant for rendering and proposing a
division that makes sense.
That is why I (rather than Ernest) have discussed only rendering related properties like bidi and default ignorable. I realise that there may be other properties which need to be considered, but I am not yet sure which these are.
I sense that you prefer to change the default properties of existing PUA characters rather than add new ones. Might it be sensible to adjust the properties in one of the PUA planes but leave the other one untouched? Has ANYONE actually defined characters in one or other of these planes, and if so, which? It would make more sense to change the default properties of a plane which no one is actually using.
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