is there a mobile *and* desktop cross-platform messaging system that doesn't require "going online"?
Is there a system to "text" my girlfriend more flexibly:
• I use iOS, she uses Android for mobile.
• I'd like either of us or both to use an internet-connected Mac with a real keyboard. Like iMessage, but cross-platform on mobile. Windows is a plus.
• Jabber is cross platform and both mobile and desktop, but as far as I can tell, you have to "go online" (log in) to a server and that connection can't be maintained 24/7 without some battery drain. (or have I got that wrong?)
• Off The Record encryption is a plus, but has that "online" problem in the implementations I've seen so far. (IM+, ChatSecure and Adium).
• An open protocol like Jabber (XMPP) is a plus.
WhatsApp, Viber & Kik seem to do login-free messaging, but only on mobile. Skype, Facebook Messenger and Google Hangouts might fill the bill. They leave privacy in someone else's hands, but I'm still interested in hearing whether they can substitute for SMS (instant-ish notification).
Plain ol' email with a fancy notification scheme
almost works but notifications take a minute or 2 instead of seconds and it "feels wrong" to use it for quick hellos.