Mouse scroll stopped working with Less in Terminal

After going through a few other answers, I've realized there is a $LESS environment variable that contains default flags.

It's empty on my other Mac and Ubuntu system, but for some reason it's equal to -FRX on this machine, and that's causing these issues:

• -X caused it to NOT accept mouse scrolls
• -F caused it to automatically exit for small files

Which is weird, since I never changed it, so it must've been modified by some other program. Anyways, manually setting it in my .rc files fixed the issue:

export LESS="-R"

Sources:

• Unix & Linux: Neither 'less' or 'less -X' clear screen on exit
• SuperUser: Less command clearing screen upon exit

In case you're using less inside git (through git log or similar):

git-config docs have the answer:

When the LESS environment variable is unset, Git sets it to FRX (if LESS environment variable is set, Git does not change it at all). If you want to selectively override Git’s default setting for LESS, you can set core.pager to e.g. less -S. This will be passed to the shell by Git, which will translate the final command to LESS=FRX less -S. The environment does not set the S option but the command line does, instructing less to truncate long lines. Similarly, setting core.pager to less -+F will deactivate the F option specified by the environment from the command-line, deactivating the "quit if one screen" behavior of less.

(emphasis mine)

TLDR:

• git is the one setting $LESS environment variable to FRX if empty
• scrolling in less doesn't work with X option set
• unset the default X option with git config --global --add core.pager "less -+X"