Patches that add hidden options are always rejected. Chrome has hidden options in about:flags but in principle they're all just hiding works in progress that will eventually be permanently on.
Patches that add options are rejected 99% of the time. Chrome has a general "no options" policy. (Here is where someone will want to comment about how that policy is dumb and is why they use Opera instead; I don't care, I'm just telling you the facts.)
Patches that permanently change the UI in the way you describe might be applied, if they're seen as strictly superior to the way things currently behave. I'm not exactly sure what you're proposing (your description seems to be describing how Chrome currently behaves? not exactly sure how constrained windows relate to that?) but your best bet is to write the patch first and then bring up a discussion of it on the development mailing list. (Writing the patch first prevents your discussion from being a discussion about vaporware.)
Sorry if I sound grumpy, I've just seen many feature proposals in my time and I'm hoping to help you not waste your time. :)