org-element: Fix holes in cache

* lisp/org-element.el (org-element--cache-stable-types): New variable.
(org-element--cache-sync): Do not store elements with missing parents
in cache.

The bug was introduced in 71c8474ae976b2d05094dcb39849dc65523c22e4.
Parsing doesn't always start from beginning of section, which means
that missing parents aren't always repaired while moving to the
current element.  Therefore, we need to remove any element from cache
with an invalid parent property.

lisp/org-element.el

@@ -5020,6 +5020,13 @@ new one."
 		       (- (max current-end bottom) offset))
 		 (incf (aref org-element--cache-status 3) offset))))))))))
 
+(defconst org-element--cache-stable-types
+  '(center-block drawer dynamic-block headline inlinetask property-drawer
+		 quote-block special-block)
+  "List of stable greater elements types.
+Stable elements are elements that don't need to be removed from
+cache when their contents only are modified.")
+
 (defun org-element--cache-sync (buffer)
   "Synchronize cache with recent modification in BUFFER.
 Elements ending before modification area are kept in cache.
@@ -5051,68 +5058,108 @@ removed from the cache."
 		 ;;
 		 ;; Such a conflict can only occur if shifted key hash
 		 ;; hasn't been processed by `maphash' yet.
-		 (unless (zerop offset)
-		   (let* ((conflictp (consp (caar value)))
-			  (value-to-shift (if conflictp (cdr value) value)))
-		     ;; Shift element part.
-		     (org-element--cache-shift-positions (car value-to-shift) offset)
-		     ;; Shift objects part.
-		     (dolist (object-data (cdr value-to-shift))
-		       (incf (car object-data) offset)
-		       (dolist (successor (nth 1 object-data))
-			 (incf (cdr successor) offset))
-		       (dolist (object (cddr object-data))
-			 (org-element--cache-shift-positions object offset)))
-		     ;; Shift key-value pair.
-		     (let* ((new-key (+ key offset))
-			    (new-value (gethash new-key org-element--cache)))
-		       ;; Put new value to shifted key.
-		       ;;
-		       ;; If one already exists, do not overwrite it:
-		       ;; store it as the car of a cons cell instead,
-		       ;; and handle it when `maphash' reaches
-		       ;; NEW-KEY.
-		       ;;
-		       ;; If there is no element stored at NEW-KEY or
-		       ;; if NEW-KEY is going to be removed anyway
-		       ;; (i.e., it is before END), just store new
-		       ;; value there and make sure it will not be
-		       ;; processed again by storing NEW-KEY in
-		       ;; NEW-KEYS.
-		       (puthash new-key
-				(if (and new-value (> new-key end))
-				    (cons value-to-shift new-value)
-				  (push new-key new-keys)
-				  value-to-shift)
-				org-element--cache)
-		       ;; If current value contains two elements, car
-		       ;; should be the new value, since cdr has been
-		       ;; shifted already.
-		       (if conflictp
-			   (puthash key (car value) org-element--cache)
-			 (remhash key org-element--cache))))))
+		 (let* ((conflictp (consp (caar value)))
+			(value-to-shift (if conflictp (cdr value) value)))
+		   (cond
+		    ;; If an elements is missing one of its parents,
+		    ;; remove it from cache.  In a middle of
+		    ;; a conflict take care not to remove already
+		    ;; shifted element.
+		    ((catch 'remove
+		       (let ((parent (car value-to-shift)))
+			 (while (setq parent
+				      (org-element-property :parent parent))
+			   (cond
+			    ((<= (org-element-property :contents-begin parent)
+				 beg)
+			     (unless (memq (org-element-type parent)
+					   org-element--cache-stable-types)
+			       (throw 'remove t)))
+			    ((<= (org-element-property :begin parent) end)
+			     (throw 'remove t))))
+			 ;; No missing parent: Proceed with shifting.
+			 nil))
+		     (if conflictp (puthash key (car value) org-element--cache)
+		       (remhash key org-element--cache)))
+		    ;; No offset: no need to shift.
+		    ((zerop offset))
+		    (t
+		     (let* ((conflictp (consp (caar value)))
+			    (value-to-shift (if conflictp (cdr value) value)))
+		       ;; Shift element part.
+		       (org-element--cache-shift-positions
+			(car value-to-shift) offset)
+		       ;; Shift objects part.
+		       (dolist (object-data (cdr value-to-shift))
+			 (incf (car object-data) offset)
+			 (dolist (successor (nth 1 object-data))
+			   (incf (cdr successor) offset))
+			 (dolist (object (cddr object-data))
+			   (org-element--cache-shift-positions object offset)))
+		       ;; Shift key-value pair.
+		       (let* ((new-key (+ key offset))
+			      (new-value (gethash new-key org-element--cache)))
+			 ;; Put new value to shifted key.
+			 ;;
+			 ;; If one already exists, do not overwrite
+			 ;; it: store it as the car of a cons cell
+			 ;; instead, and handle it when `maphash'
+			 ;; reaches NEW-KEY.
+			 ;;
+			 ;; If there is no element stored at NEW-KEY
+			 ;; or if NEW-KEY is going to be removed
+			 ;; anyway (i.e., it is before END), just
+			 ;; store new value there and make sure it
+			 ;; will not be processed again by storing
+			 ;; NEW-KEY in NEW-KEYS.
+			 (puthash new-key
+				  (if (and new-value (> new-key end))
+				      (cons value-to-shift new-value)
+				    (push new-key new-keys)
+				    value-to-shift)
+				  org-element--cache)
+			 ;; If current value contains two elements,
+			 ;; car should be the new value, since cdr has
+			 ;; been shifted already.
+			 (if conflictp
+			     (puthash key (car value) org-element--cache)
+			   (remhash key org-element--cache))))))))
 		;; Remove every element between BEG and END, since
 		;; this is where changes happened.
 		((>= key beg) (remhash key org-element--cache))
-		;; Preserve any element ending before BEG.  If it
-		;; overlaps the BEG-END area, remove it.
+		;; From now on, element starts before changes.
 		(t
 		 (let ((element (car value)))
-		   (if (>= (org-element-property :end element) beg)
-		       (remhash key org-element--cache)
-		     ;; Special case: footnote definitions and plain
-		     ;; lists can end with blank lines.  Modifying
-		     ;; those can also alter last element inside.  We
-		     ;; must therefore remove them from cache.
-		     (let ((parent (org-element-property :parent element)))
-		       (when (and parent (eq (org-element-type parent) 'item))
-			 (setq parent (org-element-property :parent parent)))
-		       (when (and (memq (org-element-type parent)
-					'(footnote-definition plain-list))
-				  (>= (org-element-property :end parent) beg)
-				  (= (org-element-property :contents-end parent)
-				     (org-element-property :end element)))
-			 (remhash key org-element--cache))))))))
+		   (cond
+		    ;; Element ended before actual buffer
+		    ;; modifications.  Remove it only if any of its
+		    ;; parents is or will be removed from cache.
+		    ((< (org-element-property :end element) beg)
+		     (catch 'remove
+		       (let ((parent element))
+			 (while (setq parent
+				      (org-element-property :parent parent))
+			   (cond
+			    ((> (org-element-property :contents-end parent) end)
+			     (unless (memq (org-element-type parent)
+					   org-element--cache-stable-types)
+			       (throw 'remove
+				      (remhash key org-element--cache))))
+			    ((>= (org-element-property :end parent) beg)
+			     (throw 'remove (remhash key org-element--cache)))))
+			 ;; No missing parent: Keep element.
+			 t)))
+		    ;; Preserve stable greater elements (or verse
+		    ;; blocks) when changes are limited to their
+		    ;; contents only.
+		    ((let ((contents-end
+			    (org-element-property :contents-end element))
+			   (type (org-element-type element)))
+		       (and contents-end
+			    (> contents-end end)
+			    (or (memq type org-element--cache-stable-types)
+				(eq type 'verse-block)))))
+		    (t (remhash key org-element--cache)))))))
 	   org-element--cache)
 	  ;; Signal cache as up-to-date.
 	  (org-element--cache-cancel-changes))))))