fp_TextRun coalescing weirdness

Could someone (martin, tf?) explain me why the following happens:

1. Type something like "lalala l" (ie, some text, a space, and 1 more
char). At this point, this is all rendered as 1 fp_TextRun, and hence 1
call to gr_UnixPangoGraphics::renderChars() is made.

2. Then type 1 more char, so you get something like: "lalala la". Now
all of a sudden for the last "a" a new fp_TextRun is created, and thus
the string is drawn in 2 separate runs.

Could someone enlighten me if this is a bug or intended behavior? Fixes
if it is a bug would be really welcome as well.

Small patch attached so you can see this happening.

Thanks,
  Marc