xz wrote:

Many text-editing (major) modes besides Text mode call text-mode-hook,
including Plain Tex, LaTex, and SliTex modes (try `M-x
apropos-documentation RET text-mode-hook RET'). And the various Tex
modes then call tex-mode-hook, and then their specific mode hook
(e.g. plain-tex-mode-hook).

Since longlines-mode toggles Long Lines (minor) mode, having the
function on both text-mode-hook and tex-mode-hook turns it on and then
off in a Tex buffer.

So if you only want it on in Tex modes, put longlines-mode on
tex-mode-hook only. Or if you want it on in all text editing modes
(including Tex modes), put longlines-mode on text-mode-hook only.

If you'd rather avoid having to remember all that, it's better to use a
function on those hooks that unconditionally turns it on. Many minor
modes have turn-on-foo-mode and turn-off-foo-mode convenience functions
for that purpose, but you can fake it with an anonymous function:

(add-hook 'text-mode-hook (lambda () (longlines-mode 1)))
(add-hook 'tex-mode-hook (lambda () (longlines-mode 1)))

or define your own convenience function and use that instead:

(defun turn-on-longlines-mode ()
"Turn on Long Lines mode."
(longlines-mode 1))
(add-hook 'text-mode-hook 'turn-on-longlines-mode)
(add-hook 'tex-mode-hook 'turn-on-longlines-mode)