fix(display): keep top borders within the mounted line grid

ebox-state-contract.el

@@ -403,6 +403,9 @@ rather than relying on an implicit default.")
     (:symbol ebox-native-status--diagnosis
      :reason buffer-local-native-status-diagnostic
      :evidence status-buffer-presentation-only)
+    (:symbol ebox--saved-overline-margin
+     :reason buffer-local-native-display-preference
+     :evidence mode-lifetime-preference-restoration-not-render-generation-state)
     (:symbol ebox-surface--observation-context
      :reason dynamically-bound-publication-observation
      :evidence let-bound-for-one-public-ebox-call)

ebox.el

@@ -4787,14 +4787,30 @@ Examples:
                  (memq (car entry) ebox--scroll-overridden-minor-modes))
                minor-mode-overriding-map-alist)))
 
+(defvar-local ebox--saved-overline-margin nil
+  "Original local-binding flag and value of `overline-margin' while mounted.")
+
 ;;;###autoload
 (define-minor-mode ebox-buffer-mode
   "Minor mode for interactive ebox buffers."
   :lighter " Ebox"
   :keymap ebox-scroll-map
   (if ebox-buffer-mode
-      (ebox--install-scroll-map-overrides)
-    (ebox--clear-scroll-map-overrides)))
+      (progn
+        (unless ebox--saved-overline-margin
+          (setq ebox--saved-overline-margin
+                (cons (local-variable-p 'overline-margin) overline-margin)))
+        ;; Native overlines otherwise add ascent to whichever row currently
+        ;; contains a top border.  Paint inside the measured line grid so
+        ;; moving a positioned border cannot move the surrounding rows.
+        (setq-local overline-margin 0)
+        (ebox--install-scroll-map-overrides))
+    (ebox--clear-scroll-map-overrides)
+    (when ebox--saved-overline-margin
+      (if (car ebox--saved-overline-margin)
+          (setq-local overline-margin (cdr ebox--saved-overline-margin))
+        (kill-local-variable 'overline-margin))
+      (setq ebox--saved-overline-margin nil))))
 
 (defun ebox--render-observer-option (options)
   "Validate OPTIONS and return `(PRESENT . OBSERVER)'."

tests/ebox-display-tests.el

@@ -5,6 +5,37 @@
 (require 'ert)
 (require 'ebox)
 
+(ert-deftest ebox-display-border-metrics-restore-on-unmount ()
+  "Mounted borders keep the line grid without leaking redisplay settings."
+  (let ((default-margin (default-value 'overline-margin)))
+    (dolist (local-margin '(nil 7))
+      (with-temp-buffer
+        (when local-margin (setq-local overline-margin local-margin))
+        (let ((input (ebox-build '(box :border "red" "Border"))))
+          (ebox-render-to-buffer (current-buffer) input)
+          (should (local-variable-p 'overline-margin))
+          (should (zerop overline-margin))
+          ;; Remounting must not replace the original setting with zero.
+          (ebox-render-to-buffer (current-buffer) input)
+          (should (zerop overline-margin))
+          (ebox-unmount-buffer (current-buffer))
+          (should (eq (local-variable-p 'overline-margin) (and local-margin t)))
+          (should (= overline-margin (or local-margin default-margin))))))
+    (should (= (default-value 'overline-margin) default-margin))))
+
+(ert-deftest ebox-display-failed-mount-preserves-border-metrics ()
+  "An unsuccessful initial publication must leave native metrics alone."
+  (with-temp-buffer
+    (setq-local overline-margin 7)
+    (let ((tp--surface-publication-step-function
+           (lambda (step _surface)
+             (when (eq step 'client-state) (error "Reject initial mount")))))
+      (should-error
+       (ebox-render-to-buffer (current-buffer)
+         (ebox-build '(box :border "red" "Border")))))
+    (should (= overline-margin 7))
+    (should-not ebox-buffer-mode)))
+
 (ert-deftest ebox-display-invalid-input-preserves-windows ()
   "Invalid input must fail before changing the caller's window layout."
   (save-window-excursion