fix(ai-term): stop F9 toggle shrinking the agent window each cycle

The F9 toggle captured the agent window's body-height and replayed it as
body-lines. Body-height subtracts the mode line's pixel height, which differs
between an active and an inactive mode line; the agent is captured active but
redisplayed inactive, so under a theme whose mode-line-inactive is shorter than
a text line the window lost ~1 line per toggle.

Capture and replay total-height for the vertical axis instead, via the renamed
cj/window-replay-size. Total-height is identical active or inactive and has no
mode-line-pixel dependence, so the round-trip is a fixed point. Width keeps
body-width (total-width has the position-dependent divider problem that total-
height does not). The shared lib fix covers the F12 terminal toggle too.

The shrink only manifests in a GUI frame, so it is not reproducible in the
batch harness; the unit tests pin the new total-height contract.

4 files changed, 62 insertions, 39 deletions

diff --git a/modules/ai-term.el b/modules/ai-term.el
index 6a1284aad..55728a3c4 100644
--- a/modules/ai-term.el
+++ b/modules/ai-term.el
@@ -445,21 +445,28 @@ the \"the displayed buffer changes\" bug.  Falls back to the buffer-list
 MRU when nil or when the remembered buffer has been killed.")
 
 (defvar cj/--ai-term-last-size nil
-  "Last user-chosen body size for the AI-term display.
+  "Last user-chosen size for the AI-term display.
 
 Positive integer: body-columns when `cj/--ai-term-last-direction'
-is right or left, body-lines when below or above.  nil means use
+is right or left, total-lines when below or above.  nil means use
 the host-aware default from `cj/--ai-term-default-size' (a float
-fraction).
-
-Body size, not total size, because total-width includes the
-right-edge divider when the window has a right sibling but excludes
-it when the window is at the frame edge.  Capturing total-width
-from a rightmost agent (no divider) and replaying into a middle
-position (with divider) leaves the body 1 column short -- visible
-as 1 col of the sibling buffer peeking through where agent should
-have ended.  Body-width is divider-independent and matches what the
-user actually sees.
+fraction).  See `cj/window-replay-size' for the per-axis capture.
+
+The axis choice is asymmetric.  Width captures body-width, not
+total-width: total-width includes the right-edge divider when the
+window has a right sibling but excludes it at the frame edge, so
+capturing total-width from a rightmost agent (no divider) and
+replaying into a middle position (with divider) leaves the body 1
+column short.  Body-width is divider-independent.
+
+Height captures total-height, not body-height: every window has
+exactly one mode line regardless of position, so total-height has
+no divider-position problem, and total-height is the same whether
+the window is active or inactive.  Body-height would subtract the
+mode line's pixel height, which differs between an active and an
+inactive (theme-shrunk) mode line -- capturing body-height active
+and replaying it inactive then re-measuring active drifts the
+window down by ~1 line per toggle (the F9 shrink bug, 2026-06-20).
 
 Absolute values rather than fractions because
 `display-buffer-in-direction' interprets a float `window-width' /
diff --git a/modules/cj-window-geometry-lib.el b/modules/cj-window-geometry-lib.el
index 4c0662124..4484a1d15 100644
--- a/modules/cj-window-geometry-lib.el
+++ b/modules/cj-window-geometry-lib.el
@@ -42,21 +42,34 @@ fails to span the full height."
      ((not spans-full-height) (if (= top root-top) 'above 'below))
      (t (or default 'right)))))
 
-(defun cj/window-body-size (window direction)
-  "Return WINDOW's body size on the axis matching DIRECTION.
+(defun cj/window-replay-size (window direction)
+  "Return WINDOW's size to capture for geometry replay, on DIRECTION's axis.
 
 Returns body-width (columns) when DIRECTION is right or left.
-Returns body-height (lines) when DIRECTION is below or above.
-
-Body size, not total size, is the right thing to capture for
-geometry replay: total-width includes the right-side divider when
-the window has a right sibling but excludes it at the frame edge,
-so a captured rightmost window replayed into a middle position
-would leave the body 1 col short.  Body size is divider-
-independent and matches what the user actually sees."
+Returns total-height (lines) when DIRECTION is below or above.
+
+The axis choice is deliberately asymmetric, for two different reasons:
+
+- Width: body-width, not total-width.  Total-width includes the right-side
+  divider when the window has a right sibling but excludes it at the frame
+  edge, so a captured rightmost window replayed into a middle position would
+  leave the body 1 col short.  Body-width is divider-independent and matches
+  what the user sees.
+
+- Height: total-height, not body-height.  Every window carries exactly one
+  mode line regardless of position, so total-height has no analog of the
+  divider-position problem -- it is position-independent.  Body-height does
+  NOT work here: it subtracts the mode line's *pixel* height, which differs
+  between an active (full-height) and an inactive (theme-shrunk) mode line.
+  Capturing body-height while the window is active and replaying it while the
+  window is displayed inactive then re-measuring active drifts the value down
+  by ~1 line per toggle whenever the inactive mode line is shorter than a text
+  line (e.g. a theme that sets `mode-line-inactive' to a sub-line height).
+  Total-height is identical active or inactive, so the capture/replay
+  round-trip is a fixed point."
   (if (memq direction '(right left))
       (window-body-width window)
-    (window-body-height window)))
+    (window-total-height window)))
 
 (defun cj/cardinal-to-edge-direction (direction)
   "Map cardinal DIRECTION to its `display-buffer-in-direction' edge variant.
diff --git a/modules/cj-window-toggle-lib.el b/modules/cj-window-toggle-lib.el
index ba91f5a40..175a1d958 100644
--- a/modules/cj-window-toggle-lib.el
+++ b/modules/cj-window-toggle-lib.el
@@ -44,7 +44,7 @@ No-op when WINDOW is nil or not live."
       (if (or (null allowed) (memq dir allowed))
           (progn
             (set direction-var dir)
-            (set size-var (cj/window-body-size window dir)))
+            (set size-var (cj/window-replay-size window dir)))
         (set direction-var default-direction)
         (set size-var nil)))))
 
@@ -59,10 +59,12 @@ DEFAULT-SIZE when the stored values are nil.  The cardinal direction
 is mapped to its frame-edge variant via
 `cj/cardinal-to-edge-direction' so the new buffer always lands at
 the same frame edge regardless of the selected window.  An integer
-size is wrapped in a `(body-columns . N)' / `(body-lines . N)' cons
-so `display-buffer-in-direction' sets the body explicitly,
-divider-independent.  A float size passes through as a fraction of
-the new window's parent.
+size is wrapped per axis: a width size as a `(body-columns . N)'
+cons (divider-independent body width), a height size as a plain
+integer total-line count.  Height uses total rather than body so the
+capture/replay round-trip is immune to the mode line's pixel height
+(see `cj/window-replay-size').  A float size passes through as a
+fraction of the new window's parent.
 
 Caller-supplied ALIST entries for direction, window-width, or
 window-height are stripped before delegating to
@@ -74,15 +76,15 @@ placement; the remaining alist entries are passed through."
          (edge-direction (or (cj/cardinal-to-edge-direction direction)
                              (cj/cardinal-to-edge-direction default-direction)))
          (size (or stored-size default-size))
-         (size-key (if (memq direction '(right left))
-                       'window-width
-                     'window-height))
-         (body-tag (if (memq direction '(right left))
-                       'body-columns
-                     'body-lines))
-         (size-value (if (integerp size)
-                         (cons body-tag size)
-                       size))
+         (width-axis (memq direction '(right left)))
+         (size-key (if width-axis 'window-width 'window-height))
+         ;; A width integer is a body-column count (divider-independent); a
+         ;; height integer is a plain total-line count (mode-line-pixel-
+         ;; independent -- see `cj/window-replay-size').  Floats pass through.
+         (size-value (cond
+                      ((not (integerp size)) size)
+                      (width-axis (cons 'body-columns size))
+                      (t size)))
          (filtered (cl-remove-if
                     (lambda (cell)
                       (memq (car-safe cell)
diff --git a/modules/term-config.el b/modules/term-config.el
index 33f54d75a..0a7991409 100644
--- a/modules/term-config.el
+++ b/modules/term-config.el
@@ -313,8 +313,9 @@ Symbol: right, left, or below.  `above' is never stored.  nil means use the
 default `below' for F12's traditional bottom split.")
 
 (defvar cj/--term-toggle-last-size nil
-  "Last user-chosen body size for the F12 terminal display.
-Positive integer: body-cols (right/left) or body-lines (below/above).
+  "Last user-chosen size for the F12 terminal display.
+Positive integer: body-cols (right/left) or total-lines (below/above) -- see
+`cj/window-replay-size' for why the vertical axis uses total, not body.
 nil means fall back to `cj/term-toggle-window-height' as a fraction.")
 
 (defun cj/--term-toggle-buffer-p (buffer)