Where is it used like that? I don't see drawing_state.transform.m11 used anywhere by itself.
You asked about using only m11 component, and I pointed out where an idea is coming from. Screenshots from the referenced bug report seem to confirm that it works well for at least an application it was tested with.
FIXME should stay unconditionally, because we don't use any aspect of it, like dashes or join type.
Correct way I imagine is to generate a stroked geometry that would be taking into account everything style has configured, together with current state. Obviously I don't expect this to happen in your patch.
Yes, the patch has limited functionality, and apparently doesn't pretend to implement support for all kinds of stroke styles. An obvious thing that the patch aims to fix is the stroke width, and according to my testing it works pretty well. Should I just drop it, or the patch could be improved and accepted?