Fourth step of attachment 226844 [details].
We need to use italic correction for correct placement scripts attached to displaystyle operators.
The special rendering added by attachment 226844 [details] is not updated after dynamic change, though.
Created attachment 226906 [details]
Created attachment 227421 [details]
Created attachment 228837 [details]
Created attachment 232575 [details]
This still does not work very well. With the current setup with anonymous box, I suspect we would need to embed the subscript inside yet another anonymous box, so that we can attach a negative margin-start. That would make the handling of tree modification even more complicated. There is probably a better solution, if we refactor the layout code to position boxes more easily. For now, I'm unassigning myself from this bug.
This issue is fixed on Alex's MathMLLayout branch, after the RenderMathMLScripts refactoring.
Fixed by the patch for bug 153918.