diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1ac0719..2ce1345 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -26,7 +26,7 @@ jobs:
         run: |
           emacs -Q --batch -L . \
             --eval '(setq byte-compile-error-on-warn t)' \
-            -f batch-byte-compile ekp.el ekp-utils.el ekp-hyphen.el
+            -f batch-byte-compile ekp.el ekp-utils.el ekp-hyphen.el ekp-region.el
       - name: Run ERT suite (C-module tests auto-skip)
         run: tests/run-tests.sh emacs
 
diff --git a/ekp-region.el b/ekp-region.el
new file mode 100644
index 0000000..20109cf
--- /dev/null
+++ b/ekp-region.el
@@ -0,0 +1,312 @@
+;;; ekp-region.el --- Buffer-level justification for ekp -*- lexical-binding: t; -*-
+
+;; Author: Kinney Zhang
+;; Keywords: wp, convenience
+
+;;; Commentary:
+
+;; Interactive layer over the ekp string API.
+;;
+;; - `ekp-justify-region' / `ekp-unjustify-region': justify buffer text
+;;   in place.  Unjustification is a pure structural transform driven by
+;;   the text properties the renderer leaves behind (`ekp-glue',
+;;   `ekp-soft-break', `ekp-soft-hyphen', `ekp-soft-trail'), so the
+;;   original text — including whitespace runs stripped at line breaks —
+;;   is recovered exactly, even after the justified text was edited.
+;;
+;; - `ekp-auto-justify-mode': keeps the whole buffer justified to the
+;;   window width.  Re-flows (debounced) when the window width changes,
+;;   and incrementally re-justifies only the edited paragraphs after
+;;   edits, so large buffers stay responsive (unchanged paragraphs hit
+;;   the ekp paragraph cache).
+
+;;; Code:
+
+(require 'ekp)
+(require 'cl-lib)
+
+(defvar ekp-auto-justify-mode)
+
+(defgroup ekp-region nil
+  "Buffer-level justification built on ekp."
+  :group 'text
+  :prefix "ekp-")
+
+(defcustom ekp-region-margin-pixel 2
+  "Pixels subtracted from the window body width when justifying.
+A small safety margin that keeps justified lines from being wrapped
+by the display engine due to rounding."
+  :type 'natnum)
+
+(defcustom ekp-auto-justify-resize-delay 0.15
+  "Seconds to debounce window-resize re-flows in `ekp-auto-justify-mode'."
+  :type 'number)
+
+(defcustom ekp-auto-justify-edit-delay 0.3
+  "Idle seconds before edited paragraphs are re-justified."
+  :type 'number)
+
+(defvar ekp-region--inhibit nil
+  "Non-nil while ekp-region is modifying the buffer itself.")
+
+(defvar-local ekp-region--auto-width nil
+  "Pixel width the buffer is currently auto-justified to.")
+
+(defvar-local ekp-region--resize-timer nil)
+(defvar-local ekp-region--edit-timer nil)
+(defvar-local ekp-region--dirty nil
+  "Pending edited regions, as a list of (BEG-MARKER . END-MARKER).")
+
+;;;; Width
+
+(defun ekp-region--window-pixel (&optional window)
+  "Usable text width in pixels of WINDOW (default: selected window)."
+  (max 1 (- (window-body-width window t) ekp-region-margin-pixel)))
+
+;;;; Pure string transforms
+
+(defun ekp-region--split-hard (string)
+  "Split justified STRING on hard newlines (those without `ekp-soft-break')."
+  (let ((parts nil) (start 0) (i 0) (len (length string)))
+    (while (< i len)
+      (when (and (eq (aref string i) ?\n)
+                 (not (get-text-property i 'ekp-soft-break string)))
+        (push (substring string start i) parts)
+        (setq start (1+ i)))
+      (setq i (1+ i)))
+    (push (substring string start) parts)
+    (nreverse parts)))
+
+(defun ekp-region--justify-string (text pixel)
+  "Return TEXT justified to PIXEL with exact-recovery markers.
+Hard newlines are preserved one-to-one.  Whitespace-only paragraphs
+(which the string API would empty out) survive as hidden text."
+  (let* ((paras (split-string text "\n"))
+         (cores (cl-remove-if #'string-blank-p paras))
+         (out (and cores
+                   (ekp-region--split-hard
+                    (ekp-pixel-justify (string-join cores "\n") pixel)))))
+    (unless (= (length out) (length cores))
+      (error "ekp-region: paragraph count mismatch (%d vs %d)"
+             (length out) (length cores)))
+    (string-join
+     (mapcar (lambda (p)
+               (if (string-blank-p p)
+                   (ekp--hide-string p)
+                 (pop out)))
+             paras)
+     "\n")))
+
+(defun ekp-region--pos-for-offset (string offset)
+  "Physical position in justified STRING for logical OFFSET.
+Glue characters count for the length of the original text they
+replaced (their `ekp-glue' value), soft breaks for their payload,
+soft hyphens for nothing; everything else (including hidden text)
+is one logical character."
+  (let ((i 0) (len (length string)))
+    (while (and (< i len) (> offset 0))
+      (let ((glue (get-text-property i 'ekp-glue string)))
+        (cond
+         (glue
+          (setq offset (- offset (length glue))))
+         ((get-text-property i 'ekp-soft-hyphen string))
+         ((and (eq (aref string i) ?\n)
+               (get-text-property i 'ekp-soft-break string))
+          (setq offset (- offset (length (get-text-property
+                                          i 'ekp-soft-break string)))))
+         (t (setq offset (1- offset)))))
+      (setq i (1+ i)))
+    i))
+
+;;;; Commands
+
+;;;###autoload
+(defun ekp-justify-region (beg end &optional pixel)
+  "Justify the text between BEG and END to PIXEL width.
+PIXEL defaults to the window text width (see `ekp-region-margin-pixel');
+interactively, a numeric prefix argument supplies it explicitly.
+Already-justified text is unjustified first, so the command is
+idempotent and can re-flow to a new width."
+  (interactive
+   (list (region-beginning) (region-end)
+         (and current-prefix-arg (prefix-numeric-value current-prefix-arg))))
+  (setq pixel (or pixel (ekp-region--window-pixel)))
+  (let ((beg (copy-marker (min beg end)))
+        (end (copy-marker (max beg end) t))
+        (ekp-region--inhibit t)
+        (inhibit-read-only t))
+    (unwind-protect
+        (atomic-change-group
+          ;; Re-flow support: strip previous justification first.
+          (when (text-property-not-all beg end 'ekp-justified nil)
+            (ekp-unjustify-region beg end))
+          (let* ((text (buffer-substring beg end))
+                 (justified (ekp-region--justify-string text pixel))
+                 (point-offset (and (>= (point) beg) (< (point) end)
+                                    (- (point) beg))))
+            (unless (equal-including-properties text justified)
+              (goto-char beg)
+              (delete-region beg end)
+              (insert justified)
+              (when point-offset
+                (goto-char (+ beg (ekp-region--pos-for-offset
+                                   justified point-offset)))))
+            (add-text-properties beg end (list 'ekp-justified pixel))))
+      (set-marker beg nil)
+      (set-marker end nil))))
+
+;;;###autoload
+(defun ekp-unjustify-region (beg end)
+  "Restore the logical text between BEG and END.
+Removes synthesized glue and soft hyphens, replaces soft line breaks
+with the whitespace they swallowed, and re-exposes hidden paragraph
+tails.  Text the user typed into the justified region is preserved."
+  (interactive "r")
+  (let ((end-m (copy-marker (max beg end) t))
+        (ekp-region--inhibit t)
+        (inhibit-read-only t))
+    (unwind-protect
+        (save-excursion
+          (goto-char (min beg end))
+          (while (< (point) end-m)
+            (let* ((pos (point))
+                   (glue (get-text-property pos 'ekp-glue)))
+              (cond
+               (glue
+                (delete-region pos (1+ pos))
+                (when (stringp glue) (insert glue)))
+               ((get-text-property pos 'ekp-soft-hyphen)
+                (delete-region pos (1+ pos)))
+               ((and (eq (char-after pos) ?\n)
+                     (get-text-property pos 'ekp-soft-break))
+                (let ((payload (get-text-property pos 'ekp-soft-break)))
+                  (delete-region pos (1+ pos))
+                  (insert payload)))
+               ((get-text-property pos 'ekp-hidden)
+                (remove-text-properties pos (1+ pos)
+                                        '(ekp-hidden nil display nil))
+                (forward-char 1))
+               (t (forward-char 1)))))
+          (remove-text-properties (min beg end) end-m '(ekp-justified nil)))
+      (set-marker end-m nil))))
+
+;;;; Auto-justify minor mode
+
+(defun ekp-region--para-bounds (marker-pair)
+  "Hard-paragraph bounds containing MARKER-PAIR, as (BEG . END)."
+  (let ((b (marker-position (car marker-pair)))
+        (e (marker-position (cdr marker-pair))))
+    (save-excursion
+      (goto-char (max (point-min) (min b (point-max))))
+      (while (and (> (point) (point-min))
+                  (let ((prev (1- (point))))
+                    (not (and (eq (char-after prev) ?\n)
+                              (not (get-text-property prev 'ekp-soft-break))))))
+        (forward-char -1))
+      (setq b (point))
+      (goto-char (max (point-min) (min e (point-max))))
+      (while (and (< (point) (point-max))
+                  (not (and (eq (char-after) ?\n)
+                            (not (get-text-property (point) 'ekp-soft-break)))))
+        (forward-char 1))
+      (cons b (point)))))
+
+(defun ekp-region--merge-regions (regions)
+  "Merge overlapping or adjacent (BEG . END) REGIONS."
+  (let ((sorted (sort regions (lambda (a b) (< (car a) (car b)))))
+        merged)
+    (dolist (r sorted)
+      (if (and merged (<= (car r) (cdr (car merged))))
+          (setcdr (car merged) (max (cdr (car merged)) (cdr r)))
+        (push (cons (car r) (cdr r)) merged)))
+    (nreverse merged)))
+
+(defun ekp-region--after-change (beg end _len)
+  "Record the edit between BEG and END for incremental re-justification."
+  (when (and ekp-auto-justify-mode (not ekp-region--inhibit))
+    (push (cons (copy-marker beg) (copy-marker end)) ekp-region--dirty)
+    (when (timerp ekp-region--edit-timer)
+      (cancel-timer ekp-region--edit-timer))
+    (setq ekp-region--edit-timer
+          (run-with-idle-timer ekp-auto-justify-edit-delay nil
+                               #'ekp-region--flush-dirty (current-buffer)))))
+
+(defun ekp-region--flush-dirty (buffer)
+  "Re-justify the paragraphs of BUFFER touched by recent edits."
+  (when (buffer-live-p buffer)
+    (with-current-buffer buffer
+      (when (and ekp-auto-justify-mode ekp-region--dirty ekp-region--auto-width)
+        (let* ((pairs (prog1 ekp-region--dirty (setq ekp-region--dirty nil)))
+               ;; Convert all bounds to markers before the first
+               ;; re-justification shifts later positions.
+               (regions (mapcar (lambda (r)
+                                  (cons (copy-marker (car r))
+                                        (copy-marker (cdr r) t)))
+                                (ekp-region--merge-regions
+                                 (mapcar #'ekp-region--para-bounds pairs)))))
+          (dolist (r regions)
+            (ekp-justify-region (car r) (cdr r) ekp-region--auto-width)
+            (set-marker (car r) nil)
+            (set-marker (cdr r) nil))
+          (dolist (p pairs)
+            (set-marker (car p) nil)
+            (set-marker (cdr p) nil)))))))
+
+(defun ekp-region--on-resize (window-or-frame)
+  "Debounced re-flow after WINDOW-OR-FRAME changed size.
+Buffer-local members of `window-size-change-functions' receive the
+window showing the buffer — and are not guaranteed to run with that
+buffer current — so resolve both explicitly."
+  (let ((win (cond ((windowp window-or-frame) window-or-frame)
+                   ((framep window-or-frame)
+                    (get-buffer-window (current-buffer) window-or-frame))
+                   (t (get-buffer-window (current-buffer))))))
+    (when (window-live-p win)
+      (with-current-buffer (window-buffer win)
+        (when ekp-auto-justify-mode
+          (let ((w (ekp-region--window-pixel win)))
+            (when (and ekp-region--auto-width (/= w ekp-region--auto-width))
+              (when (timerp ekp-region--resize-timer)
+                (cancel-timer ekp-region--resize-timer))
+              (setq ekp-region--resize-timer
+                    (run-with-timer ekp-auto-justify-resize-delay nil
+                                    #'ekp-region--reflow
+                                    (current-buffer) w)))))))))
+
+(defun ekp-region--reflow (buffer width)
+  "Re-justify all of BUFFER to WIDTH."
+  (when (buffer-live-p buffer)
+    (with-current-buffer buffer
+      (when ekp-auto-justify-mode
+        (setq ekp-region--auto-width width)
+        (ekp-justify-region (point-min) (point-max) width)))))
+
+;;;###autoload
+(define-minor-mode ekp-auto-justify-mode
+  "Keep the buffer pixel-justified to the window width.
+Re-flows when the window width changes and re-justifies edited
+paragraphs incrementally.  Designed for reading and previewing;
+the buffer text is restored exactly when the mode is turned off."
+  :lighter " EKP"
+  (if ekp-auto-justify-mode
+      (progn
+        (setq ekp-region--auto-width
+              (ekp-region--window-pixel (get-buffer-window)))
+        (ekp-justify-region (point-min) (point-max) ekp-region--auto-width)
+        (add-hook 'window-size-change-functions #'ekp-region--on-resize nil t)
+        (add-hook 'after-change-functions #'ekp-region--after-change nil t))
+    (remove-hook 'window-size-change-functions #'ekp-region--on-resize t)
+    (remove-hook 'after-change-functions #'ekp-region--after-change t)
+    (when (timerp ekp-region--resize-timer)
+      (cancel-timer ekp-region--resize-timer))
+    (when (timerp ekp-region--edit-timer)
+      (cancel-timer ekp-region--edit-timer))
+    (setq ekp-region--resize-timer nil
+          ekp-region--edit-timer nil
+          ekp-region--dirty nil
+          ekp-region--auto-width nil)
+    (ekp-unjustify-region (point-min) (point-max))))
+
+(provide 'ekp-region)
+
+;;; ekp-region.el ends here
diff --git a/ekp.el b/ekp.el
index 4dc83f6..bf53fc0 100644
--- a/ekp.el
+++ b/ekp.el
@@ -1425,41 +1425,25 @@ Each line's glues: [0 glue1 glue2 ... trailing-space]."
   (and box (not (string-empty-p box))
        (or (string-blank-p box) (= (string-width box) 0))))
 
-(defun ekp--interleave (list1 list2)
-  "Interleave elements of LIST1 and LIST2."
-  (let (result)
-    (while (or list1 list2)
-      (when list1 (push (pop list1) result))
-      (when list2 (push (pop list2) result)))
-    (nreverse result)))
-
-(defun ekp--combine-glues-and-boxes (glues boxes)
-  "Combine GLUES (n+1 elements) and BOXES (n elements) into string."
-  (let* ((glues (append glues nil))
-         (last-glue (car (last glues)))
-         (glues (butlast glues))
-         (boxes (append boxes nil)))
-    (if (= (length glues) (length boxes))
-        (string-join (append (ekp--interleave glues boxes)
-                             (list last-glue)))
-      (error "Glues count (%d) must equal boxes count (%d) + 1"
-             (1+ (length glues)) (length boxes)))))
-
 (defun ekp--strip-line-spaces (line-boxes line-glues
                                           &optional strip-leading strip-trailing)
   "Strip leading/trailing space boxes from LINE-BOXES based on flags.
 STRIP-LEADING / STRIP-TRAILING: strip space boxes at that edge.
-Returns (stripped-boxes . adjusted-glues).
+LINE-GLUES is treated as an opaque list of n+1 glue values kept in
+sync with the boxes.  Returns (kept-boxes kept-glues nlead ntrail)
+where NLEAD / NTRAIL count the boxes stripped at each edge.
 
 The stripped widths are NOT redistributed: the DP already excluded
 these space-box runs from its line metrics, so the remaining boxes
 plus distributed glues already fill the target width exactly."
-  (let* ((boxes (append line-boxes nil))
-         (glues (append line-glues nil)))
+  (let ((boxes (append line-boxes nil))
+        (glues (append line-glues nil))
+        (nlead 0) (ntrail 0))
     (when (> (length boxes) 0)
       ;; Strip trailing space boxes (if requested)
       (when strip-trailing
         (while (and boxes (ekp--box-space-p (car (last boxes))))
+          (setq ntrail (1+ ntrail))
           (setq boxes (butlast boxes))
           ;; Remove second-to-last glue (the one before the trailing
           ;; space box); keep the last glue (line's trailing filler).
@@ -1468,55 +1452,140 @@ plus distributed glues already fill the target width exactly."
       ;; Strip leading space boxes (if requested)
       (when strip-leading
         (while (and boxes (ekp--box-space-p (car boxes)))
+          (setq nlead (1+ nlead))
           (setq boxes (cdr boxes))
           ;; Remove the second glue (the one after the leading glue)
           (when (> (length glues) 1)
             (setq glues (cons (car glues) (cddr glues)))))))
-    (cons (vconcat boxes) glues)))
+    (list boxes glues nlead ntrail)))
+
+(defun ekp--box-offsets (string boxes)
+  "Locate each of BOXES in STRING; return a vector of (START . END).
+Boxes are in order and separated only by characters the tokenizer
+dropped (whitespace runs, zero-width breakers), so a sequential
+leftmost scan aligns them unambiguously."
+  (let ((offsets (make-vector (length boxes) nil))
+        (p 0) (i 0))
+    (dolist (box boxes)
+      (let ((blen (length box)))
+        (while (not (eq t (compare-strings string p (+ p blen) box 0 blen)))
+          (setq p (1+ p)))
+        (aset offsets i (cons p (+ p blen)))
+        (setq p (+ p blen))
+        (setq i (1+ i))))
+    offsets))
+
+(defun ekp--hide-string (string)
+  "Return STRING marked `ekp-hidden' and displayed as nothing."
+  (if (string-empty-p string)
+      string
+    (propertize string 'ekp-hidden t 'display "")))
+
+(defun ekp--render-glue (pixel payload)
+  "Render a glue of PIXEL width that replaced original text PAYLOAD.
+Zero-width glue renders as the hidden PAYLOAD itself, so no original
+character is ever dropped."
+  (cond
+   ((> pixel 0)
+    (propertize " " 'display `(space :width (,pixel)) 'ekp-glue payload))
+   (t (ekp--hide-string payload))))
 
 (defun ekp--hyphen-for-box (box)
-  "Return a hyphen string styled like the end of BOX."
+  "Return a hyphen string styled like the end of BOX.
+The `ekp-soft-hyphen' property marks it as synthesized, so
+`ekp-unjustify-region' can strip it structurally."
   (let ((props (and (> (length box) 0)
                     (text-properties-at (1- (length box)) box))))
-    (if props (apply #'propertize "-" props) "-")))
+    (apply #'propertize "-" 'ekp-soft-hyphen t props)))
 
 (defun ekp--pixel-justify (string line-pixel)
-  "Justify single-paragraph STRING to LINE-PIXEL."
-  (let* ((boxes (ekp--boxes string))
+  "Justify single-paragraph STRING to LINE-PIXEL.
+
+The output is lossless with respect to STRING:
+- synthesized spacing carries an `ekp-glue' property whose value is
+  the original text it replaced (usually a whitespace run),
+- soft line breaks are newlines whose `ekp-soft-break' property holds
+  the original text swallowed around the break,
+- original text outside any visible line (paragraph-edge whitespace)
+  survives as zero-display `ekp-hidden' text,
+- break hyphens carry `ekp-soft-hyphen'.
+`ekp-unjustify-region' inverts all four structurally."
+  (let* ((boxes (append (ekp--boxes string) nil))
+         (offsets (ekp--box-offsets string boxes))
          (breaks (ekp-line-breaks string line-pixel))
          (num (length breaks))
          (lines-glues (ekp-line-glues string line-pixel))
          (hyphen-positions (ekp--hyphen-positions string))
-         (start 0) strings)
+         (start 0)
+         ;; (rendered-text first-box-idx last-box-idx) per visible line
+         (lines nil))
     (dotimes (i num)
       (let* ((end (nth i breaks))
              (line-boxes (cl-subseq boxes start end))
-             (line-glues-raw (mapcar #'ekp-pixel-spacing
-                                     (aref lines-glues i)))
+             (glue-pixels (append (aref lines-glues i) nil))
              ;; Strip space boxes:
              ;; - First line (i=0): keep leading spaces (indentation)
              ;; - Other lines: strip leading spaces (break artifacts)
              ;; - All lines: strip trailing spaces
              (is-first-line (= i 0))
-             (stripped (ekp--strip-line-spaces line-boxes line-glues-raw
+             (stripped (ekp--strip-line-spaces line-boxes glue-pixels
                                                (not is-first-line)
                                                t))
-             (line-boxes (car stripped))
-             (line-glues (cdr stripped))
+             (kept (nth 0 stripped))
+             (kept-glues (nth 1 stripped))
+             (first-idx (+ start (nth 2 stripped)))
              ;; Check if last box of this line needs hyphen
              (need-hyphen
               (and (< i (1- num))  ; not last line
                    (ekp--hyphenate-p hyphen-positions (1- end)))))
-        (when (and need-hyphen (> (length line-boxes) 0))
-          (let ((last-idx (1- (length line-boxes))))
-            (aset line-boxes last-idx
-                  (concat (aref line-boxes last-idx)
-                          (ekp--hyphen-for-box (aref line-boxes last-idx))))))
-        (when (> (length line-boxes) 0)
-          (push (ekp--combine-glues-and-boxes line-glues line-boxes)
-                strings))
+        (when kept
+          (let ((parts nil) (idx first-idx) (glues kept-glues) (n 0))
+            (dolist (box kept)
+              (push (ekp--render-glue
+                     (pop glues)
+                     (if (> idx first-idx)
+                         (substring string
+                                    (cdr (aref offsets (1- idx)))
+                                    (car (aref offsets idx)))
+                       ;; leading glue of a line is always 0px and
+                       ;; replaces nothing; edge text is handled by
+                       ;; soft breaks / hidden runs below
+                       ""))
+                    parts)
+              (push box parts)
+              (setq idx (1+ idx) n (1+ n)))
+            (when need-hyphen
+              (push (ekp--hyphen-for-box (car (last kept))) parts))
+            ;; trailing filler glue (synthesized, replaces nothing)
+            (push (ekp--render-glue (car glues) "") parts)
+            (push (list (apply #'concat (nreverse parts))
+                        first-idx (+ first-idx n -1))
+                  lines)))
         (setq start end)))
-    (mapconcat 'identity (nreverse strings) "\n")))
+    (setq lines (nreverse lines))
+    (if (null lines)
+        ;; Defensive: no visible box at all (blank paragraphs are
+        ;; filtered before this function).
+        (ekp--hide-string string)
+      (let* ((first-line (car lines))
+             (last-line (car (last lines)))
+             (parts (list (ekp--hide-string
+                           (substring string 0
+                                      (car (aref offsets (nth 1 first-line)))))))
+             (prev nil))
+        (dolist (line lines)
+          (when prev
+            (push (propertize "\n" 'ekp-soft-break
+                              (substring string
+                                         (cdr (aref offsets (nth 2 prev)))
+                                         (car (aref offsets (nth 1 line)))))
+                  parts))
+          (push (nth 0 line) parts)
+          (setq prev line))
+        (push (ekp--hide-string
+               (substring string (cdr (aref offsets (nth 2 last-line)))))
+              parts)
+        (apply #'concat (nreverse parts))))))
 
 (defun ekp--validate-width (line-pixel)
   "Signal a user error unless LINE-PIXEL is a positive integer."
diff --git a/readme.md b/readme.md
index 81c2c61..8f19e22 100644
--- a/readme.md
+++ b/readme.md
@@ -64,6 +64,31 @@ engines produce **identical output**; Elisp is the always-available
 fallback.  If the module on disk is older than the Elisp code expects,
 loading refuses with a message asking you to rebuild.
 
+## Interactive Use (buffer & region)
+
+`ekp-region.el` turns the string API into buffer-level commands:
+
+```elisp
+(require 'ekp-region)
+```
+
+- `M-x ekp-justify-region` — justify the region to the window text
+  width (with a numeric prefix argument, to that many pixels).
+- `M-x ekp-unjustify-region` — restore the original text **exactly**,
+  including collapsed whitespace runs.  Justification is lossless: every
+  synthesized space, soft line break, and soft hyphen carries the
+  original text it replaced, so restoring is a structural transform that
+  also works after you edited the justified text.
+- `M-x ekp-auto-justify-mode` — keep the whole buffer justified to the
+  window width.  Re-flows (debounced by
+  `ekp-auto-justify-resize-delay`) when the window width changes, and
+  after edits re-justifies only the touched paragraphs
+  (`ekp-auto-justify-edit-delay`), so unchanged paragraphs hit the
+  paragraph cache.  Turning the mode off restores the buffer exactly.
+
+`ekp-region-margin-pixel` (default 2) is subtracted from the window
+width as a rounding safety margin.
+
 ## Configuration
 
 ### Hyphenation language
diff --git a/readme_zh.md b/readme_zh.md
index 6b5e4f1..ca7a421 100644
--- a/readme_zh.md
+++ b/readme_zh.md
@@ -57,6 +57,26 @@ cd ekp_c && make        # 需要 C11 编译器,产出 ekp.dylib/.so/.dll
 Elisp 与 C 两个引擎的输出**完全一致**;Elisp 是永远可用的后备。若磁盘
 上的模块版本旧于 Elisp 代码的要求,加载会拒绝并提示重新编译。
 
+## 交互使用(buffer 与 region)
+
+`ekp-region.el` 把字符串 API 变成 buffer 级命令:
+
+```elisp
+(require 'ekp-region)
+```
+
+- `M-x ekp-justify-region` — 把选区排版到窗口文本宽度(数字前缀参数
+  可指定像素宽)。
+- `M-x ekp-unjustify-region` — **精确**还原原文,包括被折叠的连续空
+  格。排版是无损的:每个合成空隙、软换行、软连字符都携带它所替换的
+  原文,还原是纯结构变换,即使排版后又编辑过也能正确还原。
+- `M-x ekp-auto-justify-mode` — 让整个 buffer 保持按窗口宽度排版。
+  窗口宽度变化时自动重排(防抖延迟 `ekp-auto-justify-resize-delay`);
+  编辑后只重排被改动的段落(空闲延迟 `ekp-auto-justify-edit-delay`),
+  未变段落直接命中段落缓存。关闭 mode 时 buffer 精确恢复原状。
+
+`ekp-region-margin-pixel`(默认 2)是从窗口宽度中扣除的取整安全边距。
+
 ## 配置
 
 ### 断词语言
diff --git a/tests/ekp-region-tests.el b/tests/ekp-region-tests.el
new file mode 100644
index 0000000..e80726d
--- /dev/null
+++ b/tests/ekp-region-tests.el
@@ -0,0 +1,208 @@
+;;; ekp-region-tests.el --- Tests for ekp-region.el -*- lexical-binding: t; -*-
+
+;;; Commentary:
+
+;; Batch-safe ERT tests for the buffer-level justification layer.
+;; Widths are always passed explicitly, so no window is required.
+
+;;; Code:
+
+(require 'ert)
+(require 'ekp-region)
+
+(defconst ekp-region-test--samples
+  (list "简单的中文段落测试内容,排版效果应当良好稳定。"
+        "The quick brown fox jumps over the lazy dog several times today."
+        "Mixed 中英文 paragraph with  double  spaces inside and a tail   "
+        "para one\n\npara two 混排 content here\nthird para"
+        "  leading indent 段落内容 preserved intact"
+        "短\n\n\n多个空段落之间的内容")
+  "Logical texts covering CJK, Latin, mixed, blanks, indent, tails.")
+
+(defconst ekp-region-test--widths '(30 80 200 400)
+  "Pixel widths from emergency-narrow to comfortable.")
+
+(defmacro ekp-region-test--with-text (text &rest body)
+  "Run BODY in a temp buffer containing TEXT."
+  (declare (indent 1))
+  `(with-temp-buffer
+     (insert ,text)
+     ,@body))
+
+;;;; Roundtrip exactness
+
+(ert-deftest ekp-region-test-roundtrip-exact ()
+  "justify + unjustify restores text and properties exactly."
+  (dolist (text ekp-region-test--samples)
+    (dolist (w ekp-region-test--widths)
+      (ekp-region-test--with-text text
+        (ekp-justify-region (point-min) (point-max) w)
+        (ekp-unjustify-region (point-min) (point-max))
+        (should (equal-including-properties (buffer-string) text))))))
+
+(ert-deftest ekp-region-test-roundtrip-propertized ()
+  "Roundtrip preserves user text properties."
+  (let ((text (concat (propertize "加粗的中文开头内容" 'face 'bold)
+                      " plain middle part "
+                      (propertize "italic tail words" 'face 'italic))))
+    (dolist (w '(60 250))
+      (ekp-region-test--with-text text
+        (ekp-justify-region (point-min) (point-max) w)
+        (ekp-unjustify-region (point-min) (point-max))
+        (should (equal-including-properties (buffer-string) text))))))
+
+(ert-deftest ekp-region-test-hard-newlines-preserved ()
+  "Hard newline count survives justification."
+  (ekp-region-test--with-text "a 段落 one\n\nb 段落 two\nc 段落 three"
+    (ekp-justify-region (point-min) (point-max) 100)
+    (let ((hard 0))
+      (goto-char (point-min))
+      (while (search-forward "\n" nil t)
+        (unless (get-text-property (match-beginning 0) 'ekp-soft-break)
+          (setq hard (1+ hard))))
+      (should (= hard 3)))))
+
+;;;; Justified-state invariants
+
+(ert-deftest ekp-region-test-justified-marked ()
+  "Justified region carries the ekp-justified width property."
+  (ekp-region-test--with-text "中文内容需要标记属性验证正确性"
+    (ekp-justify-region (point-min) (point-max) 120)
+    (should (eq (get-text-property (point-min) 'ekp-justified) 120))
+    (should-not (text-property-not-all (point-min) (point-max)
+                                       'ekp-justified 120))))
+
+(ert-deftest ekp-region-test-rejustify-idempotent ()
+  "Justifying at a new width equals a fresh justification at that width."
+  (let ((text "The idempotence check 中英混排 must hold across widths."))
+    (let (fresh)
+      (ekp-region-test--with-text text
+        (ekp-justify-region (point-min) (point-max) 150)
+        (setq fresh (buffer-string)))
+      (ekp-region-test--with-text text
+        (ekp-justify-region (point-min) (point-max) 300)
+        (ekp-justify-region (point-min) (point-max) 150)
+        (should (equal-including-properties (buffer-string) fresh))))))
+
+;;;; Edit robustness
+
+(ert-deftest ekp-region-test-edit-then-unjustify ()
+  "Text typed into a justified buffer survives unjustification."
+  (ekp-region-test--with-text "abcdef ghijkl 中文内容 mnopqr stuvwx"
+    (ekp-justify-region (point-min) (point-max) 80)
+    ;; Insert inside the first word: physical == logical there.
+    (goto-char (+ (point-min) 2))
+    (insert "XY")
+    (ekp-unjustify-region (point-min) (point-max))
+    (should (equal (buffer-string)
+                   "abXYcdef ghijkl 中文内容 mnopqr stuvwx"))))
+
+(ert-deftest ekp-region-test-point-stable ()
+  "Point returns to its logical position after a roundtrip."
+  (ekp-region-test--with-text "abcdef ghijkl mnopqr stuvwx yzabcd"
+    (goto-char (+ (point-min) 9))       ; inside "ghijkl"
+    (ekp-justify-region (point-min) (point-max) 60)
+    (ekp-unjustify-region (point-min) (point-max))
+    (should (= (point) (+ (point-min) 9)))))
+
+;;;; Auto-justify mode
+
+(defmacro ekp-region-test--with-mode (text width &rest body)
+  "Enable `ekp-auto-justify-mode' on TEXT at WIDTH, run BODY, disable."
+  (declare (indent 2))
+  `(ekp-region-test--with-text ,text
+     (cl-letf (((symbol-function 'ekp-region--window-pixel)
+                (lambda (&optional _) ,width)))
+       (ekp-auto-justify-mode 1)
+       (unwind-protect
+           (progn ,@body)
+         (ekp-auto-justify-mode -1)))))
+
+(ert-deftest ekp-region-test-mode-roundtrip ()
+  "Enabling then disabling the mode restores the buffer exactly."
+  (let ((text "first paragraph 内容 aaa bbb ccc\nsecond paragraph 内容 ddd"))
+    (ekp-region-test--with-mode text 150
+      (should ekp-region--auto-width)
+      (should (get-text-property (point-min) 'ekp-justified)))
+    ;; body ran; with-mode disabled the mode on exit — verify restore
+    (ekp-region-test--with-text text
+      (cl-letf (((symbol-function 'ekp-region--window-pixel)
+                 (lambda (&optional _) 150)))
+        (ekp-auto-justify-mode 1)
+        (ekp-auto-justify-mode -1)
+        (should (equal-including-properties (buffer-string) text))))))
+
+(ert-deftest ekp-region-test-mode-incremental-edit ()
+  "Edits re-justify only the touched paragraph, content stays correct."
+  (let ((text "aaa bbb ccc ddd eee fff\nggg hhh iii jjj kkk lll")
+        (calls nil))
+    (ekp-region-test--with-mode text 100
+      (let ((orig (symbol-function 'ekp-justify-region)))
+        (cl-letf (((symbol-function 'ekp-justify-region)
+                   (lambda (b e &optional px)
+                     (push (cons (marker-position (copy-marker b))
+                                 (marker-position (copy-marker e)))
+                           calls)
+                     (funcall orig b e px))))
+          ;; Edit inside paragraph 1.
+          (goto-char (+ (point-min) 4))
+          (insert "zz")
+          (should ekp-region--dirty)
+          (ekp-region--flush-dirty (current-buffer))
+          ;; Exactly one incremental call, confined before the hard \n.
+          (should (= (length calls) 1))
+          (let ((hard-nl (save-excursion
+                           (goto-char (point-min))
+                           (catch 'nl
+                             (while (search-forward "\n" nil t)
+                               (unless (get-text-property (match-beginning 0)
+                                                          'ekp-soft-break)
+                                 (throw 'nl (match-beginning 0))))))))
+            (should (<= (cdar calls) hard-nl)))))
+      ;; Logical text after disable = original with the edit applied.
+      (ekp-auto-justify-mode -1)
+      (should (equal (buffer-string)
+                     "aaa zzbbb ccc ddd eee fff\nggg hhh iii jjj kkk lll"))
+      ;; re-enable so with-mode's cleanup disable is a no-op state-wise
+      (ekp-auto-justify-mode 1))))
+
+(ert-deftest ekp-region-test-resize-hook-window-arg ()
+  "The resize hook handles its WINDOW argument and foreign current buffer.
+Regression: buffer-local `window-size-change-functions' members get
+the displaying WINDOW, with an arbitrary buffer current."
+  (let ((text "resize hook 检查 aaa bbb ccc ddd eee fff"))
+    (ekp-region-test--with-mode text 200
+      (let ((buf (current-buffer))
+            (win (selected-window)))
+        (set-window-buffer win buf)
+        (cl-letf (((symbol-function 'ekp-region--window-pixel)
+                   (lambda (&optional _) 120)))
+          ;; simulate redisplay: window argument, unrelated buffer current
+          (with-temp-buffer
+            (ekp-region--on-resize win)))
+        (with-current-buffer buf
+          (should (timerp ekp-region--resize-timer))
+          (cancel-timer ekp-region--resize-timer)
+          ;; run what the timer would have run
+          (ekp-region--reflow buf 120)
+          (should (= ekp-region--auto-width 120)))))))
+
+(ert-deftest ekp-region-test-mode-reflow-width ()
+  "Reflow to a new width matches a fresh justification at that width."
+  (let ((text "reflow 检查 aaa bbb ccc ddd eee fff ggg hhh")
+        fresh)
+    (ekp-region-test--with-text text
+      (ekp-justify-region (point-min) (point-max) 90)
+      (setq fresh (buffer-substring (point-min) (point-max))))
+    (ekp-region-test--with-mode text 200
+      (ekp-region--reflow (current-buffer) 90)
+      (should (= ekp-region--auto-width 90))
+      (let ((got (buffer-substring (point-min) (point-max))))
+        ;; ekp-justified was written at two widths; ignore that prop
+        (remove-text-properties 0 (length got) '(ekp-justified nil) got)
+        (remove-text-properties 0 (length fresh) '(ekp-justified nil) fresh)
+        (should (equal-including-properties got fresh))))))
+
+(provide 'ekp-region-tests)
+
+;;; ekp-region-tests.el ends here
diff --git a/tests/run-tests.sh b/tests/run-tests.sh
index 10cbcd9..bb12abe 100755
--- a/tests/run-tests.sh
+++ b/tests/run-tests.sh
@@ -7,4 +7,5 @@ ROOT="$(cd "$(dirname "$0")/.." && pwd)"
 
 exec "$EMACS" -Q --batch -L "$ROOT" \
      -l "$ROOT/tests/ekp-tests.el" \
+     -l "$ROOT/tests/ekp-region-tests.el" \
      -f ert-run-tests-batch-and-exit