missing glyphs in the default font are often not being replaced by fallback glyphs from other fonts

Normally, when my default font doesn't contain a character (say from a non-Latin script), it finds that character in another font and displays it.

I've noticed that in Qt apps, this isn't happening in all cases. For example, if I go to https://secure.wikimedia.org/wikipedia/en/wiki/Tamil_script in Konqueror I get boxes for the Tamil letters, whereas in Firefox they are displayed. I have reproduced this problem with various other scripts which are not in my default font. Some characters (for example Chinese) do display correctly, however, even though they are not in my default font.

This is not uniquely a Konqueror problem. It affects all Qt applications (Kate, Kmail, etc). I know that in previous Qt/KDE versions, this was not the case. I can't pinpoint exactly when this problem first occurred, but I believe it happened when I upgraded to 4.6.3.

The most important example that I have found is this one:
http://en.wikipedia.org/wiki/Syriac_alphabet#Summary_table
Notice that the names of the letters appear as boxes, but the letters themselves appear because they have been explicitly defined with 'font-family' in the HTML code.

I'm guessing that the selection of a fallback font is buggy in that it chooses fallbacks which do not contain the required characters. I should add that if the default font contains the necessary glyphs, they display correctly.

Reproducible: Always

Steps to Reproduce:
Go to http://en.wikipedia.org/wiki/Uyghur_alphabet#Present_situation

Actual Results:
Note that several Arabic characters do not appear (ie Qt is using a fallback font that contains some of the missing letters but not all of them).

Expected Results:
On Firefox and Chrome, a different fallback font is used which covers all the missing letters.