Proposal:
1. Change the General Category for the following characters from Cf to Po (Punctuation Other), and thereby changing the status to not be Default_Ignorable_Code_Points:
FFF9 INTERLINEAR ANNOTATION ANCHOR FFFA INTERLINEAR ANNOTATION SEPARATOR FFFB INTERLINEAR ANNOTATION TERMINATOR
2. Clarify that some characters shown with a dotted-box in the code charts are not Cf characters. (This is already the case for the Object Replacement Character.)
Discussion:
The Default_Ignorable_Code_Points are those that should be ignored in display if they are not supported. There are several reasons for this change, but the primary reason is that if these characters are unsupported, the best representation is not to silently omit them, but to supply a visual indication that there is something at that position. Otherwise the annotating text can be misinterpreted as part of the original text stream.
If the General Category of the characters were Po, instead, the characters would not be Default Ignorable, and the black box display (if interpretation of the characters themselves is unsupported) would be the default.
This change is within the allowable changes according to the Unicode Stability Policies. While this does involve a change to a normative property, as with many other cases the effective normative change is only to APIs that purport to return the General Category value (or dependent property values) for the version of Unicode where this change is made. This is because the actual appearance of a Cf or Po character is not constrained by the standard, although there are strong guidelines in Chapter 5. In particular, this does not change the combining status, which would affect normalization and canonical equivalence.
Note: While the Musical symbols below are similar in some aspects to the annotation characters, their usage is highly dependent on specialized support for music symbols, often not part of natural language support. So the proposal does not include these characters. However, if there are arguments for including these characters also, that should made a response to this public issue.
1D173 MUSICAL SYMBOL BEGIN BEAM 1D174 MUSICAL SYMBOL END BEAM 1D175 MUSICAL SYMBOL BEGIN TIE 1D176 MUSICAL SYMBOL END TIE 1D177 MUSICAL SYMBOL BEGIN SLUR 1D178 MUSICAL SYMBOL END SLUR 1D179 MUSICAL SYMBOL BEGIN PHRASE 1D17A MUSICAL SYMBOL END PHRASE