fix(nov): rework the EPUB reading-width layout

`cj/nov--text-width-for-window' computed the target column as a percentage of `(window-body-width)'. But body width is the column count *after* the display margins. `cj/nov-update-layout' runs from `window-configuration-change-hook': it sets `visual-fill-column''s margins, which changes the body width, which fires the hook, which re-runs the layout against the now-narrower body, and so on. It's a shrinking feedback loop that bottoms out at `cj/nov-min-text-width' (40 columns) no matter what `cj/nov-margin-percent' is. That's why the column was a thin strip regardless of the margin setting.

The width is now computed from the window's *natural* column count (body width plus any margins already set), so re-running the layout is idempotent. The margin math moved into a pure `cj/nov--text-width' helper, which is what the unit tests drive, and there's a regression test that the result is the same whether or not margins are already in place.

Also:
- `+'/`=' (`cj/nov-widen-text') and `-'/`_' (`cj/nov-narrow-text') step `cj/nov-margin-percent' by `cj/nov-margin-step' and re-lay-out, reporting the new percentage. `cj/nov-margin-percent' is now clamped to 0..25, so the text column runs from 50% (the floor) to 100% (the full window).
- `cj/nov-margin-percent' default is 12 (≈76% text) for a comfortable starting width.
- `cj/nov-apply-preferences' re-renders the document at the end again. `b3b537f' removed that on the theory `visual-fill-column' would re-trigger the render. The first page came up off-center until a manual resize, so it's back.
- `cj/nov-update-layout' is now a command.

The visible result (a ~75% centered column on first open, `+`/`-` to adjust) needs a restart to confirm. The tests cover the width math and clamping, idempotency, the adjust commands and their keybindings, the command status, and the re-render.

2 files changed, 180 insertions, 42 deletions

diff --git a/modules/calibredb-epub-config.el b/modules/calibredb-epub-config.el
index 43032381..2e02d1a9 100644
--- a/modules/calibredb-epub-config.el
+++ b/modules/calibredb-epub-config.el
@@ -49,6 +49,7 @@
 (declare-function cj/open-file-with-command "system-utils" (command))
 (declare-function visual-fill-column-mode "visual-fill-column" (&optional arg))
 (declare-function visual-fill-column--adjust-window "visual-fill-column" ())
+(declare-function nov-render-document "nov" ())
 
 ;; -------------------------- CalibreDB Ebook Manager --------------------------
 
@@ -89,13 +90,18 @@
 
 ;; ------------------------------ Nov Epub Reader ------------------------------
 
-(defvar cj/nov-margin-percent 25
-  "Percentage of window width to use as margins on each side when reading epubs.
-For example, 25 means 25% left margin + 25% right margin, with 50% for text.")
+(defvar cj/nov-margin-percent 12
+  "Percent of the window's natural width used as a margin on each side in epubs.
+12 leaves about 76% of the columns for text.  Clamped to 0..25, so the text
+column runs from 50% (margin 25) to 100% (margin 0) of the window.
+Adjust it live with `cj/nov-widen-text' and `cj/nov-narrow-text'.")
 
 (defvar cj/nov-min-text-width 40
   "Minimum text width in columns for Nov reading buffers.")
 
+(defvar cj/nov-margin-step 2
+  "Percentage points each `cj/nov-widen-text'/`cj/nov-narrow-text' press changes.")
+
 ;; Prevent magic-fallback-mode-alist from opening epub as archive-mode
 ;; Advise set-auto-mode to force nov-mode for .epub files before magic-fallback runs
 (defun cj/force-nov-mode-for-epub (orig-fun &rest args)
@@ -127,21 +133,35 @@ For example, 25 means 25% left margin + 25% right margin, with 50% for text.")
   (forward-paragraph)
   (recenter))
 
+(defun cj/nov--text-width (total-cols)
+  "Return the Nov text-column width for TOTAL-COLS of usable window width.
+`cj/nov-margin-percent' is clamped to 0..25 and taken off each side; the
+result is at least `cj/nov-min-text-width'."
+  (let* ((margin-percent (max 0 (min 25 cj/nov-margin-percent)))
+         (text-width-ratio (- 1.0 (* 2 (/ margin-percent 100.0)))))
+    (max cj/nov-min-text-width
+         (floor (* text-width-ratio total-cols)))))
+
 (defun cj/nov--text-width-for-window (&optional window)
   "Return preferred Nov text width for WINDOW.
-The width uses `cj/nov-margin-percent' while keeping a readable minimum and
-clamping excessive margin percentages."
+Computed from the window's natural column count -- its current body width
+plus any margins already set -- so that re-running `cj/nov-update-layout' is
+idempotent: each pass would otherwise shave the column by another margin
+fraction, since setting margins narrows the body width."
   (let* ((window (or window (get-buffer-window (current-buffer) t)))
-         (window-width (if window (window-body-width window) 80))
-         (margin-percent (max 0 (min 45 cj/nov-margin-percent)))
-         (text-width-ratio (- 1.0 (* 2 (/ margin-percent 100.0)))))
-    (max cj/nov-min-text-width
-         (floor (* text-width-ratio window-width)))))
+         (margins (and window (window-margins window)))
+         (natural-cols (if window
+                           (+ (window-body-width window)
+                              (or (car margins) 0)
+                              (or (cdr margins) 0))
+                         80)))
+    (cj/nov--text-width natural-cols)))
 
 (defun cj/nov-update-layout (&optional _frame)
-  "Recalculate Nov text layout for the current buffer.
-Suitable for `window-configuration-change-hook' or
-`window-size-change-functions'."
+  "Recalculate Nov's centered text column (width + margins) for this buffer.
+Also runs from `window-configuration-change-hook' and
+`window-size-change-functions' to stay responsive to splits and resizes."
+  (interactive)
   (when (derived-mode-p 'nov-mode)
     (when (require 'visual-fill-column nil t)
       (setq-local visual-fill-column-center-text t)
@@ -150,6 +170,25 @@ Suitable for `window-configuration-change-hook' or
       (when (bound-and-true-p visual-fill-column-mode)
         (visual-fill-column--adjust-window)))))
 
+(defun cj/--nov-adjust-margin (delta)
+  "Add DELTA to `cj/nov-margin-percent' (clamped 0..25), re-lay-out, and report.
+A positive DELTA narrows the text column; a negative DELTA widens it."
+  (setq cj/nov-margin-percent
+        (max 0 (min 25 (+ cj/nov-margin-percent delta))))
+  (cj/nov-update-layout)
+  (message "EPUB text width: %d%% of the window (margin %d%% each side)"
+           (- 100 (* 2 cj/nov-margin-percent)) cj/nov-margin-percent))
+
+(defun cj/nov-widen-text ()
+  "Give the EPUB text column more of the window, up to the full width."
+  (interactive)
+  (cj/--nov-adjust-margin (- cj/nov-margin-step)))
+
+(defun cj/nov-narrow-text ()
+  "Give the EPUB text column less of the window, down to 50%."
+  (interactive)
+  (cj/--nov-adjust-margin cj/nov-margin-step))
+
 (defun cj/nov-apply-preferences ()
   "Apply preferences after nov-mode has launched."
   (interactive)
@@ -168,7 +207,10 @@ Suitable for `window-configuration-change-hook' or
   ;; Enable visual-fill-column for centered text with margins
   (cj/nov-update-layout)
   ;; Keep centered text width responsive after splits/resizes.
-  (add-hook 'window-configuration-change-hook #'cj/nov-update-layout nil t))
+  (add-hook 'window-configuration-change-hook #'cj/nov-update-layout nil t)
+  ;; Re-render so the first page is laid out inside the margins just set;
+  ;; nov-mode's initial render ran before `nov-text-width'/visual-fill-column.
+  (nov-render-document))
 
 (defun cj/nov-open-external ()
   "Open the current EPUB with zathura."
@@ -191,6 +233,11 @@ Suitable for `window-configuration-change-hook' or
 		("<" . nov-history-back)
 		(">" . nov-history-forward)
 		("," . backward-paragraph)
+		;; +/= widen the text column, -/_ narrow it (50%..100% of the window)
+		("+" . cj/nov-widen-text)
+		("=" . cj/nov-widen-text)
+		("-" . cj/nov-narrow-text)
+		("_" . cj/nov-narrow-text)
 		;; open current EPUB with zathura (same key in pdf-view)
 		("z" . cj/nov-open-external)
 		("t" . nov-goto-toc)
diff --git a/tests/test-calibredb-epub-config.el b/tests/test-calibredb-epub-config.el
index 53d2e78f..88c4452f 100644
--- a/tests/test-calibredb-epub-config.el
+++ b/tests/test-calibredb-epub-config.el
@@ -2,54 +2,145 @@
 
 ;;; Commentary:
 ;; Focuses on project-owned helpers in calibredb-epub-config rather than
-;; CalibreDB/Nov internals.
+;; CalibreDB/Nov internals.  The Nov layout helpers get the most attention:
+;; the text-width math, the idempotency of `cj/nov-update-layout' (it must not
+;; shrink the column each time it runs), and the cold-open re-render.
 
 ;;; Code:
 
 (require 'ert)
 (require 'cl-lib)
+(require 'package)
 
+(setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
+(package-initialize)
 (add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
 (require 'calibredb-epub-config)
+(require 'nov nil t)  ; for the nov-mode-map keybinding test; harmless if absent
 
-(ert-deftest test-calibredb-epub-nov-text-width-default-window ()
-  "Normal: text width uses the configured margins against the current window."
+(declare-function cj/nov--text-width "calibredb-epub-config" (total-cols))
+
+;;; ----------------------------- cj/nov--text-width ---------------------------
+
+(ert-deftest test-calibredb-epub-nov-text-width-applies-margin ()
+  "Normal: 25% margins leave 50% of the usable columns for text."
   (let ((cj/nov-margin-percent 25)
         (cj/nov-min-text-width 40))
-    (cl-letf (((symbol-function 'get-buffer-window)
-               (lambda (&rest _) 'window))
-              ((symbol-function 'window-body-width)
-               (lambda (_) 120)))
-      (should (= 60 (cj/nov--text-width-for-window))))))
+    (should (= 60 (cj/nov--text-width 120)))))
 
 (ert-deftest test-calibredb-epub-nov-text-width-clamps-large-margin ()
-  "Boundary: excessive margins are clamped to keep a readable text column."
+  "Boundary: a margin percent above 25 is clamped to 25, so text never drops
+below 50% of the usable columns."
   (let ((cj/nov-margin-percent 80)
         (cj/nov-min-text-width 40))
-    (cl-letf (((symbol-function 'get-buffer-window)
-               (lambda (&rest _) 'window))
-              ((symbol-function 'window-body-width)
-               (lambda (_) 120)))
-      (should (= 40 (cj/nov--text-width-for-window))))))
+    (should (= 60 (cj/nov--text-width 120)))))
 
-(ert-deftest test-calibredb-epub-nov-text-width-fallback-without-window ()
-  "Boundary: a buffer without a visible window still gets a usable width."
+(ert-deftest test-calibredb-epub-nov-text-width-clamps-negative-margin ()
+  "Boundary: a negative margin percent is clamped up to 0 (text takes everything)."
+  (let ((cj/nov-margin-percent -10)
+        (cj/nov-min-text-width 40))
+    (should (= 120 (cj/nov--text-width 120)))))
+
+(ert-deftest test-calibredb-epub-nov-text-width-honours-minimum ()
+  "Boundary: a narrow window still yields at least `cj/nov-min-text-width'."
   (let ((cj/nov-margin-percent 25)
         (cj/nov-min-text-width 40))
-    (cl-letf (((symbol-function 'get-buffer-window)
-               (lambda (&rest _) nil)))
-      (should (= 40 (cj/nov--text-width-for-window))))))
+    (should (= 40 (cj/nov--text-width 50)))))
 
-(ert-deftest test-calibredb-epub-nov-text-width-clamps-negative-margin ()
-  "Boundary: a negative margin percent is clamped up to 0, so the text takes
-the full window width."
-  (let ((cj/nov-margin-percent -10)
+(ert-deftest test-calibredb-epub-nov-default-margin-gives-roughly-three-quarter-text ()
+  "Normal: the default `cj/nov-margin-percent' leaves ~3/4 of the window for text."
+  (should (= 76 (cj/nov--text-width 100))))
+
+;;; ----------------------- cj/nov--text-width-for-window ----------------------
+
+(ert-deftest test-calibredb-epub-nov-text-width-for-window-fresh ()
+  "Normal: with no margins set yet, the natural width is the body width."
+  (let ((cj/nov-margin-percent 25)
+        (cj/nov-min-text-width 40))
+    (cl-letf (((symbol-function 'get-buffer-window) (lambda (&rest _) 'win))
+              ((symbol-function 'window-body-width) (lambda (_) 120))
+              ((symbol-function 'window-margins) (lambda (_) '(nil . nil))))
+      (should (= 60 (cj/nov--text-width-for-window))))))
+
+(ert-deftest test-calibredb-epub-nov-text-width-for-window-idempotent ()
+  "Boundary: re-running with margins already set returns the same width.
+The body width is now narrower because margins were applied, but the natural
+width (body + margins) is unchanged, so the column does not shrink.  Without
+this, every layout pass would shave the column by another margin fraction."
+  (let ((cj/nov-margin-percent 25)
+        (cj/nov-min-text-width 40))
+    (cl-letf (((symbol-function 'get-buffer-window) (lambda (&rest _) 'win))
+              ((symbol-function 'window-body-width) (lambda (_) 60))
+              ((symbol-function 'window-margins) (lambda (_) '(30 . 30))))
+      (should (= 60 (cj/nov--text-width-for-window))))))
+
+(ert-deftest test-calibredb-epub-nov-text-width-for-window-no-window ()
+  "Boundary: a buffer with no visible window still gets a usable width."
+  (let ((cj/nov-margin-percent 25)
         (cj/nov-min-text-width 40))
-    (cl-letf (((symbol-function 'get-buffer-window)
-               (lambda (&rest _) 'window))
-              ((symbol-function 'window-body-width)
-               (lambda (_) 120)))
-      (should (= 120 (cj/nov--text-width-for-window))))))
+    (cl-letf (((symbol-function 'get-buffer-window) (lambda (&rest _) nil)))
+      (should (= 40 (cj/nov--text-width-for-window))))))
+
+;;; ---------------------------- cj/nov-update-layout --------------------------
+
+(ert-deftest test-calibredb-epub-nov-update-layout-is-a-command ()
+  "Normal: `cj/nov-update-layout' can be invoked with `M-x'."
+  (should (commandp #'cj/nov-update-layout)))
+
+;;; --------------------- cj/nov-widen-text / cj/nov-narrow-text ---------------
+
+(ert-deftest test-calibredb-epub-nov-adjust-margin-steps-and-clamps ()
+  "Normal/Boundary: adjusting the margin moves by DELTA, clamped to 0..25."
+  (cl-letf (((symbol-function 'message) (lambda (&rest _) nil)))
+    (let ((cj/nov-margin-percent 12))
+      (cj/--nov-adjust-margin -2)
+      (should (= 10 cj/nov-margin-percent))
+      (cj/--nov-adjust-margin 100)
+      (should (= 25 cj/nov-margin-percent))    ; 50%-text floor
+      (cj/--nov-adjust-margin -100)
+      (should (= 0 cj/nov-margin-percent)))))  ; 100%-text ceiling
+
+(ert-deftest test-calibredb-epub-nov-widen-text-decreases-margin ()
+  "Normal: `cj/nov-widen-text' gives the column more of the window."
+  (cl-letf (((symbol-function 'message) (lambda (&rest _) nil)))
+    (let ((cj/nov-margin-percent 12)
+          (cj/nov-margin-step 2))
+      (cj/nov-widen-text)
+      (should (= 10 cj/nov-margin-percent)))))
+
+(ert-deftest test-calibredb-epub-nov-narrow-text-increases-margin ()
+  "Normal: `cj/nov-narrow-text' gives the column less of the window."
+  (cl-letf (((symbol-function 'message) (lambda (&rest _) nil)))
+    (let ((cj/nov-margin-percent 12)
+          (cj/nov-margin-step 2))
+      (cj/nov-narrow-text)
+      (should (= 14 cj/nov-margin-percent)))))
+
+(ert-deftest test-calibredb-epub-nov-width-commands-are-commands ()
+  "Normal: the width-adjust commands are `M-x'-able."
+  (should (commandp #'cj/nov-widen-text))
+  (should (commandp #'cj/nov-narrow-text)))
+
+(ert-deftest test-calibredb-epub-nov-width-commands-bound-in-nov-mode-map ()
+  "Normal: +/= widen and -/_ narrow the text column in `nov-mode-map'."
+  (skip-unless (and (require 'nov nil t) (boundp 'nov-mode-map)))
+  (should (eq (keymap-lookup nov-mode-map "+") #'cj/nov-widen-text))
+  (should (eq (keymap-lookup nov-mode-map "=") #'cj/nov-widen-text))
+  (should (eq (keymap-lookup nov-mode-map "-") #'cj/nov-narrow-text))
+  (should (eq (keymap-lookup nov-mode-map "_") #'cj/nov-narrow-text)))
+
+;;; -------------------------- cj/nov-apply-preferences ------------------------
+
+(ert-deftest test-calibredb-epub-nov-apply-preferences-rerenders-document ()
+  "Normal: applying preferences re-renders the document so the first page
+lands inside the margins it just configured."
+  (let (rendered)
+    (cl-letf (((symbol-function 'nov-render-document) (lambda () (setq rendered t))))
+      (with-temp-buffer
+        (cj/nov-apply-preferences)
+        (should rendered)))))
+
+;;; ----------------------------- cj/nov-open-external -------------------------
 
 (ert-deftest test-calibredb-epub-open-external-uses-zathura ()
   "Normal: named Nov external-open command delegates to zathura."