Comment 1 for bug 353663

It's not interesting that ctrl-home generates the same key sequence inside and outside screen since that's a terminal emulator thing.

I believe the issue is that when TERM=screen, vim doesn't set the proper termcap settings for interpreting the modified home and end keys (and others). Compare :set <End> when running vim inside and outside screen. I use a vim plugin to set those options to always recognize the modifiers.