IMHO, isolate surrogates are not valid starters for combining sequences, they must remain isolate : deleting this surrogate in your text editor should not delete the following combining mark which is a separate cluster (even if that cluster is defective before the deletion as it has NO base starter) For default grapheme clusters, it would be helpful to add a rule to force a cluster break before and after any lone surogate (i.e. for grapheme cluster breaking, treat any lone character as if it were a control like NUL U+0000).
2015-10-04 15:02 GMT+02:00 Richard Wordingham < richard.wording...@ntlworld.com>: > In the absence of a specific tailoring, is the combination of a lone > surrogate and a combining mark a user-perceived character? Does a lone > surrogate constitute a user-perceived character? > > The problem I have is that because of an application-specific bug, > when I attempt to enter the sequence <U+1148F TIRHUTA LETTER KA, > U+114BA TIRHUTA SIGN CANDRABINDU>, I appear to be gettig the UTF-16 code > unit sequence <D805 DC8F D805 D805 DCBA>, which is being interpreted as > the codepoint sequence <U+1148F, U+D805, U+114BA>. > > (The problem seems to arise because I use a sequence of two key strokes > to enter candrabindu, and the application or input mechanism has to undo > the entry of a supplementary character entered in response to the first > keystroke. I've reported the problem as Bug 94753.) > > Because the lone surrogate is interpreted as the start of a > user-perceived character, I can move the cursor to between U+1148F and > U+D805. Then pressing the 'delete' key (as opposed to the 'rubout' > key) will delete the U+D805. However, if the lone surrogate plus > combining mark is a user-perceived character, then all I will be left > with is <U+1148F>. At present the offending application is treating > Tirhuta combining marks as user-perceived characters, but I suspect the > application has simply not caught up with Unicode Version 7 yet. > > Richard. >