Harbuzz seems to support Malayalam well with fonts that come with certain glyphs pre-combined. One of these fonts with the pre-combined glyphs is Meera. On the other hand there are other fonts that require the shaper to combine certain glyphs for Malayalam, for example JanaMalayalam or Kartika (which is the default font for Malayalam on Windows).
A problematic Malayalam string is UTF-16 sequence 3349 3405 3376, aka. 0x0D15, 0x0D4D, 0x0D30. Qt renders it correctly with Meera but incorrectly with JanaMalayalam and Kartika.
Google Chrome 8 renders the combined glyph correctly
Qt 4.7.1 combines it in the wrong order
Qt 4.7.2 renders it incorrectly in a different way
See attached screen shot and test application