I'm a user of Florian Balmers Notepad2, which use the Scintilla-component - the problem is reproducable in other Scintilla-based programs too, so it's very likely that this problem is a bug in Scintilla itself.
I'm using the german version of Windows XP SP3 Home, with installed support for east-asian languages (Chinese, Japanese, Korean). I use Notepad2 to edit UTF8-encoded-XML files, which contain different languages (English, German, French, Chinese, Japanese, Korean).
Now the problem is, that the Korean characters are sometimes replaced by squares and sometimes they are shown correctly - even in the same file.
While modifying a XML file, one very mysterious thing occured now:
This screenshot show that the Korean characters are shown correctly:
If I mark the "r" of "kr" the Korean characters get replaced by squares:
If I remove the selection the characters are shown correctly again.
This problem seems to occur only for Korean. Other east-asian characters (Chinese and Japanese) are shown correctly.
To help you reproducing the problem, here is the file from the screenshot:
The problem occurs in line 3197, column 151