Thanks Brogan, yes the varying postbit height is the main issue here. My limited expertise ends with fixing such thing for constant heights.
So, when you said manually editing, can you please elaborate. Is it actually possible with CSS, template edit or not? I think it might be possible with JS to calculate the height of the postbit run-time and then change the min-height of the message container (just an idea, not sure though, but I'd like to do it with CSS if possible).