E.g. with 0041 030B LATIN CAPITAL LETTER A WITH COMBINING DOUBLE ACUTE ACCENT the accent E.g. with 0041 030B LATIN CAPITAL LETTER A WITH COMBINING DOUBLE ACUTE ACCENT the accent appears at the right hand side of the letter A, not above the letter A.
If e.g. an "X" is prepended, the sequence is rendered correctly - with the exception of the COMBINING HORN which should be at the right side of the base letter.
The tested sequences are used in the following specification:
DIN SPEC 91379: Characters in Unicode for the electronic processing of names and dataexchange in Europe; with digital attachment
The output of FOP is provided in test-din-spec-sequences.fo.pdf, which is created by running TestFop.java that processes test-din-spec-sequences.fo.
Font used for testing: NotoSansMono-Regular.ttf, see https://www.google.com/get/noto/
The following patch seems to resolve the problem for my test case:
(delete "&& (unscaledWidths[index] != 0").
See also PDFBOX-4951