In QFontEngineMulti::stringToCMap() we call the primary engine's
implementation of the same function. If this engine does not support
the character in question, then it's supposed to clear the glyph array,
otherwise there may be left-over junk in the glyph array from previous
script items and the font selection algorithm will think it has already
found a match for the character corresponding to the glyph position.
The freetype engine, for instance, clears the respective entries
in the array when it gets a 0 glyph from the font engine.
In particular, this would happen when you had a ligature preceding
an item that was shaped separately. The ligature (e.g. "fi") would
set the first two slots of the glyph array, but later replace
them with a single glyph. The next item would then get an offset
of 1, i.e. pointing to the position in the glyph array where the
glyph for i was originally contained. If this was not cleared,
it would assume the primary engine supported the character. If
the character was of an unsupported writing system, then you would
get a box in place of it instead.
[ChangeLog][OS X][Text] Fixed appending text with a different
writing system and formatting to a latin ligature.
Change-Id: Id8c81cdc8e2d8994cc1a999769fcae452c4f52ae
Task-number: QTBUG-44708
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
c29c6d9003