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.

1 files changed, 119 insertions, 28 deletions

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."