Helper posted a file on the forum that can be read fine with other editors like PSPad and Notepad++, but it only shows a couple lines in CE. I'll attach the file here.

After examining the file in a hex editor, it appears that CE terminates rendering a line when it encounters a null byte. PSPad and WordPan renders a space for the null byte. I did not check Notepad++ to see what it does.

We should probably show the NULL as an "unknown byte" character...which currently is set to a ?, but I think we should probably use something else and not actually do the replacement as we currently do. If you open a file with unknown characters, they show as '?'. If you save said file with CE and reopen it in anything, the characters actually become '?' IIRC.

I would be inclined to follow suit with the others and just render a space.

Would be nice to know what program is creating that file, and why it is inserting a null byte in the middle of the file.

One point I would make is that (generally) if a file is corrupted then it has 0x00 in the corrupt sections. Using a space would allow the file to still be readable. (perhaps we should also include a warning message that the file may be corrupt to warn the user, although I would expect some debate on this particular feature)

I think the problem lies in cedtElement.cpp in the following function:

BOOL CAnalyzedText::FileLoad(LPCTSTR lpszPathName, INT nEncodingType, INT nFileFormat)

Also possibly this function:

BOOL CMemText::FileLoad(LPCTSTR lpszPathName)

Also of interest is:
cedtDoc.cpp

BOOL CCedtDoc::OnOpenDocument(LPCTSTR lpszPathName)