fix(chat): stabilize fenced code block redisplay

eca-chat.el

@@ -2763,6 +2763,16 @@ matches the region we considered."
         (setq pos next))))
   `(jit-lock-bounds ,beg . ,end))
 
+(defun eca-chat--font-lock-ensure (beg end)
+  "Fontify BEG to END without shifting visible chat windows.
+Hidden markdown markup can change display geometry after streaming.
+Preserve window starts and force a cheap redisplay update so fenced
+block layout settles before later point motion."
+  (prog1
+      (eca-chat--with-preserved-scroll
+        (font-lock-ensure beg end))
+    (force-window-update (current-buffer))))
+
 (defun eca-chat--schedule-fontify ()
   "Schedule a deferred `font-lock-ensure' for the current chat buffer.
 Cancels any previously scheduled timer.  Does nothing when
@@ -2785,7 +2795,7 @@ the new turn instead of the full chat history."
                (when (buffer-live-p buf)
                  (with-current-buffer buf
                    (setq eca-chat--fontify-timer nil)
-                   (font-lock-ensure
+                   (eca-chat--font-lock-ensure
                     (or eca-chat--last-user-message-pos (point-min))
                     (point-max))))))))))
 
@@ -3910,8 +3920,9 @@ Must be called with `eca-chat--with-current-buffer' or equivalent."
                ;; (re)fontification at end-of-stream; full-buffer
                ;; fontify is O(buffer size) and was the dominant
                ;; cost on long chats.
-               (font-lock-ensure (or eca-chat--last-user-message-pos (point-min))
-                                 (point-max))
+               (eca-chat--font-lock-ensure
+                (or eca-chat--last-user-message-pos (point-min))
+                (point-max))
                (eca-chat--refresh-copy-scopes)
                (eca-chat--set-chat-loading session nil)
                (eca-chat--refresh-progress chat-buffer)
@@ -3932,8 +3943,9 @@ Must be called with `eca-chat--with-current-buffer' or equivalent."
                ;; to the current turn so cost does not grow with
                ;; chat history; previous turns were already
                ;; fontified at their own end-of-stream.
-               (font-lock-ensure (or eca-chat--last-user-message-pos (point-min))
-                                 (point-max))
+               (eca-chat--font-lock-ensure
+                (or eca-chat--last-user-message-pos (point-min))
+                (point-max))
                (eca-chat--refresh-copy-scopes)
                ;; Table align/beautify default to scanning from
                ;; `eca-chat--last-user-message-pos' when called with

test/eca-chat-test.el

@@ -482,6 +482,35 @@ does not treat the first line as metadata.  Returns FN's value."
         (expect (current-kill 0 t)
                 :to-equal "Previous assistant response")))))
 
+(describe "eca-chat--font-lock-ensure"
+  (it "fontifies and requests a redisplay update"
+    (with-temp-buffer
+      (spy-on 'font-lock-ensure :and-return-value 'fontified)
+      (spy-on 'force-window-update)
+      (expect (eca-chat--font-lock-ensure 1 1) :to-equal 'fontified)
+      (expect 'font-lock-ensure :to-have-been-called-with 1 1)
+      (expect 'force-window-update
+              :to-have-been-called-with (current-buffer)))))
+
+(describe "eca-chat--schedule-fontify"
+  (it "uses stable fontification from the current turn"
+    (with-temp-buffer
+      (insert "abc")
+      (setq-local eca-chat--last-user-message-pos 2)
+      (setq-local eca-chat-fontify-debounce-interval 0.15)
+      (let (scheduled)
+        (cl-letf (((symbol-function 'run-with-idle-timer)
+                   (lambda (_secs _repeat fn &rest args)
+                     (setq scheduled (lambda () (apply fn args)))
+                     'test-timer)))
+          (spy-on 'eca-chat--font-lock-ensure)
+          (eca-chat--schedule-fontify)
+          (expect scheduled :to-be-truthy)
+          (funcall scheduled)
+          (expect 'eca-chat--font-lock-ensure
+                  :to-have-been-called-with 2 (point-max))
+          (expect eca-chat--fontify-timer :to-be nil))))))
+
 (describe "eca-chat--render-content"
   (describe "progress finished"
     (it "clears progress text and spinner when chat-loading is nil"
@@ -512,6 +541,31 @@ does not treat the first line as metadata.  Returns FN's value."
                       :to-have-been-called))
           (when (timerp eca-chat--spinner-timer)
             (cancel-timer eca-chat--spinner-timer))
+          (kill-buffer buf))))
+
+    (it "uses stable fontification for normal completion"
+      (let ((buf (eca-chat-test--make-prompt-buffer ""))
+            (session (make-eca--session)))
+        (unwind-protect
+            (with-current-buffer buf
+              (setq-local eca-chat--progress-text "thinking...")
+              (setq-local eca-chat--chat-loading t)
+              (setq-local eca-chat--last-user-message-pos (point-min))
+              (spy-on 'eca-chat--font-lock-ensure)
+              (spy-on 'eca-chat--align-tables)
+              (spy-on 'eca-chat--beautify-tables)
+              (spy-on 'eca-chat--refresh-progress)
+              (spy-on 'eca-chat--set-chat-loading)
+              (spy-on 'eca-chat--send-steered-prompt)
+              (spy-on 'eca-chat--send-queued-prompt)
+              (eca-chat--render-content
+               session buf "system"
+               (list :type "progress" :state "finished")
+               nil)
+              (expect 'eca-chat--font-lock-ensure
+                      :to-have-been-called-with (point-min) (point-max))
+              (expect 'eca-chat--align-tables :to-have-been-called)
+              (expect 'eca-chat--beautify-tables :to-have-been-called))
           (kill-buffer buf))))))
 
 (describe "eca-chat--transient-segment-loading"