patch 8.2.4954: inserting line breaks text property spanning two lines

Problem:    Inserting line breaks text property spanning more then one line.
Solution:   Check TP_FLAG_CONT_PREV and TP_FLAG_CONT_NEXT. (closes #10423)
diff --git a/src/textprop.c b/src/textprop.c
index 5ec983f..6fc628a 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1650,11 +1650,12 @@
     proptype_T	*pt = text_prop_type_by_id(curbuf, prop->tp_type);
     int		start_incl = (pt != NULL
 				    && (pt->pt_flags & PT_FLAG_INS_START_INCL))
-						   || (flags & APC_SUBSTITUTE);
+				|| (flags & APC_SUBSTITUTE)
+				|| (prop->tp_flags & TP_FLAG_CONT_PREV);
     int		end_incl = (pt != NULL
-				     && (pt->pt_flags & PT_FLAG_INS_END_INCL));
-		// Do not drop zero-width props if they later can increase in
-		// size.
+				      && (pt->pt_flags & PT_FLAG_INS_END_INCL))
+				|| (prop->tp_flags & TP_FLAG_CONT_NEXT);
+    // Do not drop zero-width props if they later can increase in size.
     int		droppable = !(start_incl || end_incl);
     adjustres_T res = {TRUE, FALSE};