Thought I’d post a followup on my previous post since it generated a bit of interest. First a quick summary:
- It is not an Ubuntu Dapper specific bug. With the appropriate combination of fonts and pango versions, it will exhibit itself on other Pango-enabled Firefox builds (it was verified on the Fedora build too).
- It is not a DejaVu bug, although it is one of the few fonts to exhibit the problem. The simple fact is that not many fonts provide ligature glyphs and include the required OpenType tables for them to be used.
- It isn’t a Pango bug. The ligatures are handled correctly in normal GTK applications on Dapper. The bug only occurs with Pango >= 1.12, but that is because older versions did not make use of the OpenType tables in the “basic” shaper (used for latin scripts like english).
- The bug only occurs in the Pango backend, but then the non-Pango renderer doesn’t even support ligatures. Furthermore, there are a number of languages that can’t be displayed correctly with the non-Pango renderer so it is not very appealing.
The firefox bug is only triggered in the slow, manual glyph positioning code path of the text renderer. This only gets invoked if you have non-default letter or word spacing (such as justified text). In this mode, the width of the normal glyph of the first character in the ligature seems to be used for positioning which results in the overlapping text.
It seems that the bug may be fixed in the Firefox 1.6 series, but if that fix can’t be backported easily in time for Dapper, it might be easier to switch to a different default font that doesn’t contain the ligatures (such as Bitstream Vera). That would certainly reduce the chance of the bug occurring.