Normally, Ctrl+Y would "undo the undo" and put me back where I was. But in Twitter's compose tweet dialog box, Ctrl+Y does nothing.
This is on PC, Linux, Chromium browser.
Ctrl + Shift + Z. Tested on Linux with Chrome browser.
I ran into the problem of accidentally undoing some formatted content on a Mac using Chrome. While command + z is undo, the shortcut to redo in Chrome on the Mac is control + z.