A quite short sentence here.
New sentence on the same paragraph, with more text with more text with more text with more text.
Some more text, still on the same paragraph.
This is another very very very very very very very very very very very very very very very very very very very very long sentence.

A quite short sentence here.
New sentence on the same paragraph, with more text with more
text with more text with more text.
Some more text, still on the same paragraph.
This is another very very very very very very very very very
very very very very very very very very very very very long
sentence.

    A quite short sentence here.
    New sentence on the same paragraph, with more text with
more text with more text with more text.
    Some more text, still on the same paragraph.
    This is another very very very very very very very very
very very very very very very very very very very very very
long sentence.
    \begin{equation}
        H = equation * lines + sometimes - \need{to}{be / 
long}
    \end{equation}

(require 'longlines nil t)
(add-hook 'LaTeX-mode-hook #'longlines-mode)

(defun longlines-encode-region (beg end &optional _buffer)
  "Replace each soft newline between BEG and END with exactly one space.
Hard newlines are left intact. The optional argument BUFFER exists for
compatibility with `format-alist', and is ignored."
  (save-excursion
    (let ((reg-max (max beg end))
          (mod (buffer-modified-p)))
      (goto-char (min beg end))
      ;; Changed this line to "swallow" indendation when decoding.
      (while (search-forward-regexp " *\\(\n\\) *" reg-max t)
        (let ((pos (match-beginning 1)))
          (unless (get-text-property pos 'hard)            
            (goto-char (match-end 0))   ; This line too
            (insert-and-inherit " ")
            (replace-match "" :fixedcase :literal) ; This line too
            (remove-text-properties pos (1+ pos) 'hard))))
      (set-buffer-modified-p mod)
      end)))

(defun longlines-wrap-line ()
  "If the current line needs to be wrapped, wrap it and return nil.
If wrapping is performed, point remains on the line. If the line does
not need to be wrapped, move point to the next line and return t."
  (if (and (bound-and-true-p latex-extra-mode)
           (null (latex/do-auto-fill-p)))
      (progn (forward-line 1) t)
    ;; The conditional above was added for latex equations. It relies
    ;; on the latex-extra package (on Melpa).
    (if (and (longlines-set-breakpoint)
             ;; Make sure we don't break comments.
             (null (nth 4 (parse-partial-sexp
                           (line-beginning-position) (point)))))
        (progn
          ;; This `let' and the `when' below add indentation to the
          ;; wrapped line.
          (let ((indent (save-excursion (back-to-indentation)
                                        (current-column))))
            (insert-before-markers-and-inherit ?\n)
            (backward-char 1)
            (delete-char -1)
            (forward-char 1)
            (when (> indent 0)
              (save-excursion
                (insert (make-string indent ? )))
              (setq longlines-wrap-point
                    (+ longlines-wrap-point indent))))
          nil)
      (if (longlines-merge-lines-p)
          (progn (end-of-line)
                 (if (or (prog1 (bolp) (forward-char 1)) (eolp))
                     (progn
                       (delete-char -1)
                       (if (> longlines-wrap-point (point))
                           (setq longlines-wrap-point
                                 (1- longlines-wrap-point))))
                   (insert-before-markers-and-inherit ?\s)
                   (backward-char 1)
                   (delete-char -1)
                   (forward-char 1)
                   ;; This removes whitespace added for indentation.
                   (while (eq (char-after) ? )
                     (delete-char 1)
                     (setq longlines-wrap-point
                           (1- longlines-wrap-point))))
                 nil)
        (forward-line 1)
        t))))