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.
