Restore hl-line--buffer tracking

* lisp/hl-line.el (hl-line-overlay, hl-line-overlay-buffer):
Correct replacement variable.
(hl-line--overlay): Clearer doc.
(hl-line--buffer): Nee hl-line-overlay-buffer
(hl-line-sticky-flag): Custom initialization is unfathomable.
(hl-line-mode, hl-line-unhighlight): Orthogonalize sticky.
(hl-line-highlight): Remove highlight from previous buffer.
* test/lisp/hl-line-tests.el (hl-line-sticky, hl-line-tests-verify):
(hl-line-tests-sticky-across-frames, hl-line-tests-sticky):
Test (bug#54481).

2 files changed, 107 insertions, 33 deletions

diff --git a/lisp/hl-line.el b/lisp/hl-line.el
index 70ba0fcfc28..f1c2e1ebf23 100644
--- a/lisp/hl-line.el
+++ b/lisp/hl-line.el
@@ -24,17 +24,26 @@
 
 ;;; Commentary:
 
+;;  Proper scuttling of unsticky overlays relies on `post-command-hook`
+;;  being called on a buffer switch and the stationarity of
+;;  `hl-line--buffer` across switches.  One could easily imagine
+;;  programatically defeating unsticky overlays by bypassing
+;; `post-command-hook`.
+
 ;;; Code:
 
-(make-obsolete-variable 'hl-line-overlay nil "29.1")
+(make-obsolete-variable 'hl-line-overlay 'hl-line--overlay "29.1")
 (make-obsolete-variable 'global-hl-line-overlay nil "29.1")
 (make-obsolete-variable 'global-hl-line-overlays nil "29.1")
 (make-obsolete-variable 'global-hl-line-sticky-flag nil "29.1")
-(make-obsolete-variable 'hl-line-overlay-buffer nil "29.1")
+(make-obsolete-variable 'hl-line-overlay-buffer 'hl-line--buffer "29.1")
 (make-obsolete-variable 'hl-line-range-function nil "29.1")
 
 (defvar-local hl-line--overlay nil
-  "Keep state else scan entire buffer in `post-command-hook'.")
+  "The prevailing highlighting overlay per buffer.")
+
+(defvar hl-line--buffer nil
+  "Used to track last buffer.")
 
 ;; 1. define-minor-mode creates buffer-local hl-line--overlay
 ;; 2. overlay wiped by kill-all-local-variables
@@ -68,6 +77,7 @@
   :type 'boolean
   :version "22.1"
   :group 'hl-line
+  :initialize #'custom-initialize-default
   :set (lambda (symbol value)
          (set-default symbol value)
          (unless value
@@ -100,14 +110,12 @@ Currently used in calendar/todo-mode."
         (add-hook 'post-command-hook #'hl-line-highlight nil t))
     (remove-hook 'post-command-hook #'hl-line-highlight t)
     (remove-hook 'change-major-mode-hook #'hl-line-unhighlight t)
-    (let (hl-line-sticky-flag)
-      (hl-line-unhighlight))))
+    (hl-line-unhighlight)))
 
 (defun hl-line-unhighlight ()
-  (unless hl-line-sticky-flag
-    (when hl-line--overlay
-      (delete-overlay hl-line--overlay)
-      (setq hl-line--overlay nil))))
+  (when hl-line--overlay
+    (delete-overlay hl-line--overlay)
+    (setq hl-line--overlay nil)))
 
 (defun hl-line-highlight ()
   (unless (minibufferp)
@@ -120,6 +128,12 @@ Currently used in calendar/todo-mode."
     (move-overlay hl-line--overlay
                   (line-beginning-position)
                   (line-beginning-position 2))
+    (when (and (not (eq hl-line--buffer (current-buffer)))
+               (not hl-line-sticky-flag)
+               (buffer-live-p hl-line--buffer))
+      (with-current-buffer hl-line--buffer
+        (hl-line-unhighlight)))
+    (setq hl-line--buffer (current-buffer))
     (run-hooks 'hl-line-highlight-hook)))
 
 (defun hl-line-turn-on ()
diff --git a/test/lisp/hl-line-tests.el b/test/lisp/hl-line-tests.el
index 422d4ddae7d..6bff09135b2 100644
--- a/test/lisp/hl-line-tests.el
+++ b/test/lisp/hl-line-tests.el
@@ -21,30 +21,90 @@
 (require 'ert)
 (require 'hl-line)
 
-(ert-deftest hl-line-sticky ()
-  (should hl-line-sticky-flag)
-  (with-temp-buffer
-    (let ((from-buffer (current-buffer)))
-      (hl-line-mode 1)
-      (save-excursion
-        (insert "foo"))
-      (hl-line-highlight)
-      (should (cl-some (apply-partially #'eq hl-line--overlay)
-                       (overlays-at (point))))
-      (switch-to-buffer (get-buffer-create "*scratch*"))
-      (hl-line-mode 1)
-      (save-excursion
-        (insert "bar"))
-      (hl-line-highlight)
-      (should (cl-some (apply-partially #'eq hl-line--overlay)
-                       (overlays-at (point))))
-      (should (buffer-local-value 'hl-line--overlay from-buffer))
-      (should-not (eq (buffer-local-value 'hl-line--overlay from-buffer)
-                      hl-line--overlay))
-      (customize-set-variable 'hl-line-sticky-flag nil)
-      (should hl-line--overlay)
-      (should (buffer-live-p from-buffer))
-      (should-not (buffer-local-value 'hl-line--overlay from-buffer)))))
+(defsubst hl-line-tests-verify (_label on-p)
+  (eq on-p (cl-some (apply-partially #'eq hl-line--overlay)
+                    (overlays-at (point)))))
+
+(ert-deftest hl-line-tests-sticky-across-frames ()
+  (skip-unless (display-graphic-p))
+  (customize-set-variable 'hl-line-sticky-flag t)
+  (call-interactively #'global-hl-line-mode)
+  (let ((first-frame (selected-frame))
+        (first-buffer "foo")
+        (second-buffer "bar")
+        second-frame)
+    (unwind-protect
+        (progn
+          (switch-to-buffer first-buffer)
+          (save-excursion
+            (insert (buffer-name)))
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 111 t))
+          (select-frame (setq second-frame (make-frame)))
+          (switch-to-buffer second-buffer)
+          (save-excursion
+            (insert (buffer-name)))
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 762 t))
+          (with-current-buffer first-buffer
+            (should (hl-line-tests-verify 534 t)))
+          (call-interactively #'global-hl-line-mode)
+          (should (hl-line-tests-verify 125 nil))
+          (with-current-buffer first-buffer
+            (should (hl-line-tests-verify 892 nil)))
+
+          ;; now do unsticky
+          (customize-set-variable 'hl-line-sticky-flag nil)
+          (call-interactively #'global-hl-line-mode)
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 467 t))
+          (with-current-buffer first-buffer
+            (should (hl-line-tests-verify 765 nil)))
+          (select-frame first-frame)
+          (should (equal (buffer-name) first-buffer))
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 423 t))
+          (with-current-buffer second-buffer
+            (should (hl-line-tests-verify 897 nil))))
+      (let (kill-buffer-query-functions)
+        (ignore-errors (kill-buffer first-buffer))
+        (ignore-errors (kill-buffer second-buffer))
+        (ignore-errors (delete-frame second-frame))))))
+
+(ert-deftest hl-line-tests-sticky ()
+  (customize-set-variable 'hl-line-sticky-flag t)
+  (let ((first-buffer "foo")
+        (second-buffer "bar"))
+    (unwind-protect
+        (progn
+          (switch-to-buffer first-buffer)
+          (hl-line-mode 1)
+          (save-excursion
+            (insert (buffer-name)))
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 123 t))
+          (switch-to-buffer second-buffer)
+          (hl-line-mode 1)
+          (save-excursion
+            (insert (buffer-name)))
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 56 t))
+          (with-current-buffer first-buffer
+            (should (hl-line-tests-verify 67 t)))
+
+          ;; now do unsticky
+          (customize-set-variable 'hl-line-sticky-flag nil)
+          (should (hl-line-tests-verify 234 t))
+          (with-current-buffer first-buffer
+            (should (hl-line-tests-verify 231 nil)))
+          (switch-to-buffer first-buffer)
+          (run-hooks 'post-command-hook)
+          (should (hl-line-tests-verify 257 t))
+          (with-current-buffer second-buffer
+            (should (hl-line-tests-verify 999 nil)))))
+    (let (kill-buffer-query-functions)
+      (ignore-errors (kill-buffer first-buffer))
+      (ignore-errors (kill-buffer second-buffer)))))
 
 (provide 'hl-line-tests)