When there's an invalid combining character sequence, OOo displays each
combining character in  the sequence individually on a dotted circle to clearly
indicate that there's an error. See issue 42171 and issue 42662 for detail
explanation. Currently this only works on Linux, see issue 42171, and only for
fonts with the dotted circle, see issue 42662.

However, even on Linux it works only with some fonts like Lucidasans and Norasi.
It doesn't work with the new Windows Unicode fonts like Tahoma, Microsoft San
Serif or Lucida Sans Unicode which all have the dotted circle.

Test case :-
- On Linux, load the Writer document attached. It has  three lines containing a
Thai base consonant 'gor gai', three tone marks 'mai ek' (the last two are
invalid and couldn't be displayed and should be displayed over a dotted circle),
and another 'gor gai'. See the attached screenshot.

- The first line is formatted using 'Tahoma', the second using 'Microsoft Sans
Serif' and the third using 'Lucida Sans Unicode'. There's a non-spacing black
box that overstrike the first gor-gai. There's also two dotted circles that
overstrike both gor-gai. In the first two line, the dotted-circles don't take
space because the second gor-gai is displayed next to the first one. In the
third line, they take a wide space and make the caret moving strangely. You can
try moving the caret using the left/right arrow key on the third line.

This issue should also be tested on Windows when the dotted circle feature is
working.