fix(dropdown) be sure the list is the top object on the screen

fixes #2893

src/core/lv_obj_tree.c

@@ -188,6 +188,10 @@ void lv_obj_move_to_index(lv_obj_t * obj, int32_t index)
 {
     LV_ASSERT_OBJ(obj, MY_CLASS);
 
+    if(index < 0) {
+        index = lv_obj_get_child_cnt(lv_obj_get_parent(obj)) + index;
+    }
+
     const int32_t old_index = lv_obj_get_index(obj);
 
     lv_obj_t * parent = lv_obj_get_parent(obj);

src/core/lv_obj_tree.h

@@ -97,8 +97,8 @@ void lv_obj_swap(struct _lv_obj_t * obj1, struct _lv_obj_t * obj2);
  * moves the object to the given index in its parent.
  * When used in listboxes, it can be used to sort the listbox items.
  * @param obj  pointer to the object to be moved.
- * @param index  new index in parent.
- * @note to move to the foreground: lv_obj_move_to_index(obj, 0)
+ * @param index  new index in parent. -1 to count from the back
+ * @note to move to the background: lv_obj_move_to_index(obj, 0)
  * @note to move forward (up): lv_obj_move_to_index(obj, lv_obj_get_index(obj) - 1)
  */
 void lv_obj_move_to_index(struct _lv_obj_t * obj, int32_t index);

src/widgets/lv_dropdown.c

@@ -422,6 +422,8 @@ void lv_dropdown_open(lv_obj_t * dropdown_obj)
     lv_dropdown_t * dropdown = (lv_dropdown_t *)dropdown_obj;
 
     lv_obj_add_state(dropdown_obj, LV_STATE_CHECKED);
+    lv_obj_set_parent(dropdown->list, lv_obj_get_screen(dropdown_obj));
+    lv_obj_move_to_index(dropdown->list, -1);
     lv_obj_clear_flag(dropdown->list, LV_OBJ_FLAG_HIDDEN);
 
     /*To allow styling the list*/

tests/src/test_cases/test_dropdown.c

@@ -4,15 +4,16 @@
 #include "unity/unity.h"
 #include "lv_test_indev.h"
 
-void test_dropdown_create_delete(void);
-void test_dropdown_set_options(void);
-void test_dropdown_select(void);
-void test_dropdown_click(void);
-void test_dropdown_keypad(void);
-void test_dropdown_encoder(void);
-void test_dropdown_render_1(void);
-void test_dropdown_render_2(void);
+void setUp(void)
+{
+    /* Function run before every test */
+}
 
+void tearDown(void)
+{
+    /* Function run after every test */
+    lv_obj_clean(lv_scr_act());
+}
 void test_dropdown_create_delete(void)
 {
   lv_dropdown_create(lv_scr_act());
@@ -422,6 +423,23 @@ void test_dropdown_render_2(void)
   TEST_ASSERT_EQUAL_SCREENSHOT("dropdown_2.png");
 }
 
+/* See #2893 */
+void test_dropdown_should_list_on_top(void)
+{
+    lv_obj_t * cont1 = lv_obj_create(lv_scr_act());
+    lv_obj_set_size(cont1, 200, 100);
+
+    lv_obj_t * dd = lv_dropdown_create(cont1);
+
+    lv_obj_t * cont2 = lv_obj_create(lv_scr_act());
+    lv_obj_set_size(cont2, 200, 100);
+    lv_obj_set_pos(cont2, 0, 100);
+
+    lv_dropdown_open(dd);
+    lv_obj_t * list = lv_dropdown_get_list(dd);
+    TEST_ASSERT_EQUAL_PTR(lv_scr_act(), lv_obj_get_parent(list));
+    TEST_ASSERT_EQUAL_INT(2, lv_obj_get_index(list));
+}
 
 
 #endif