Section 5.13 of the Unicode 4.0 standard - 'Rendering Nonspacing Marks' -
recommends a method called 'Show Hidden' to indicate the inability to draw an
invalid combining character sequence. It does so by drawing the base character
first and then rendering the nonspacing mark as an individual unit - with the
nonspacing mark positioned on a dotted circle. The method is implemented in OOo.
However, the implementation on Windows is currently broken, see issue 42171. The
feature only works on Linux now.

The problem is - not all fonts have this dotted circle glyph. In particular, by
far the most popular fonts used in Thailand are the Thai-specific fonts shipped
with Windows (Angsana, Browalia, Cordia, etc.).  These all lack the dotted
circle glyphs. In case the glyph is missing, there should be a fallback
mechanism. Currently OOo seems to do nothing.

Test case: 
- Currently, you have to test it on Linux. Load the attached Writer document. 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 another 'gor
gai'. See the attached screenshot.

1) The first line is formatted using the default Lucidasans, the second using
Norasi, both of them have the dotted circle. You can see that the excess marks
are shown individually on each dotted circle.

2) The third line is formatted using the Windows Thai font 'Angsana New'. You
can try other fonts that have Thai but not dotted circle (all Windows Thai
fonts). There'll be a non-spacing black box that overstrike the first gor-gai.
So you can't tell what are in the invalid sequence.

To handle this, there should be a fallback mechanism in case there's no dotted
circle glyph to use. The possibility are using the 'missing glyph shape' (white
box) like Uniscribe
(http://www.microsoft.com/typography/otfntdev/thaiot/shaping.aspx) or simply
using space. The point is to place each combining character individually so the
users can see what the error is. Therefore the fallback glyph must be spacing.

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