How to scroll in a terminal by line in GNOME Terminal 2.31.3?

On more recent versions of gnome-terminal, Shift+Ctrl+↑ and Shift+Ctrl+↓ work for scrolling by line, but I have no way of checking for 2.31.

CentOS 6 has 2.31.3; CentOS 7 has 3.14.3 (where the feature works). It is not a new feature, having been introduced in 2007:

Bug 118967 – single line scrolling with "Ctrl+Shift+ArrowUp/ArrowDown"

while 2.31.3 dates from 2010 (a noticeable delay even for the enterprise releases).

However, it does not work with CentOS 6 and incidentally, the git-commit logs for vte and gnome-terminal do not appear to mention when it was fixed. Likewise, if there were a bug-report associated with the change, one might point to when the feature was broken.

The feature also appears to be not configurable, as one might expect: there is no shortcut for this key combination in the preferences dialog.

Given all of that, there's no apparent workaround in CentOS 6, and the feature might be (still) working in CentOS 7 when you upgrade.