Prepare markers for insertions inside of a widget

Recreating child widgets without recreating the parent widget
may lead to situations where the parent widget doesn't cover its
children or buttons entirely anymore.  This bug manifests as a
faulty fontification of children or buttons, for example.
(Bug#69941)

* lisp/wid-edit.el (widget--prepare-markers-for-inside-insertion)
(widget--prepare-markers-for-outside-insertion): New functions.
(widget-default-create): Use them.

* test/lisp/wid-edit-tests.el (widget-test-insertion-at-parent-markers)
(widget-test-insertion-at-parent-markers-2): New tests.

1 files changed, 51 insertions, 0 deletions

diff --git a/test/lisp/wid-edit-tests.el b/test/lisp/wid-edit-tests.el
index c18e6d14c4c..e34aa64f8d1 100644
--- a/test/lisp/wid-edit-tests.el
+++ b/test/lisp/wid-edit-tests.el
@@ -430,4 +430,55 @@ return nil, even with a non-nil bubblep argument."
       (should-not (overlay-buffer field-overlay))
       (should-not (overlay-buffer field-end-overlay)))))
 
+;; The following two tests are for Bug#69941.  Markers need to be prepared
+;; against "inside" insertions at them.  That is, a recreated child should
+;; still be covered by the parent's :from and :to markers.
+(ert-deftest widget-test-insertion-at-parent-markers ()
+  "Test that recreating a child keeps the parent's markers covering it.
+
+Test the most common situation, where only one parent needs to be adjusted."
+  (with-temp-buffer
+    (let* ((group (widget-create 'group
+                                 :format "%v"
+                                 '(item :value 1 :format "%v")))
+           (item (car (widget-get group :children)))
+           (ofrom (marker-position (widget-get group :from)))
+           (oto (marker-position (widget-get group :to))))
+      (widget-insert "\n")
+      (widget-setup)
+      ;; Change item, without recreating the group.  This causes changes
+      ;; right at the :from and :to markers, and if they don't have
+      ;; the right type, the group's :from-:to span won't include its
+      ;; child, the item widget, anymore.
+      (widget-value-set item 2)
+      ;; The positions should be the same as they were when the group
+      ;; widget was first created.
+      (should (= ofrom (widget-get group :from)))
+      (should (= oto (widget-get group :to))))))
+
+(ert-deftest widget-test-insertion-at-parent-markers-2 ()
+  "Test that recreating a child keeps the parent's marker covering it.
+
+Test the uncommon situation in which we might need to prepare the grandparent's
+markers (and so on) as well."
+  (with-temp-buffer
+    (let* ((group (widget-create '(group
+                                   :format "%v"
+                                   (group
+                                    :format "%v"
+                                    (item :value 1 :format "%v")))))
+           (group2 (car (widget-get group :children)))
+           (item (car (widget-get group2 :children)))
+           (ofrom (marker-position (widget-get group :from)))
+           (oto (marker-position (widget-get group :to)))
+           (ofrom2 (marker-position (widget-get group2 :from)))
+           (oto2 (marker-position (widget-get group2 :to))))
+      (widget-insert "\n")
+      (widget-setup)
+      (widget-value-set item 2)
+      (should (= ofrom (widget-get group :from)))
+      (should (= oto (widget-get group :to)))
+      (should (= ofrom2 (widget-get group2 :from)))
+      (should (= oto2 (widget-get group2 :to))))))
+
 ;;; wid-edit-tests.el ends here