feat(scroll): user-defined scrollbar length using LV_STYLE_LENGTH (#7306)

Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>

docs/details/base-widget/scroll.rst

@@ -74,9 +74,27 @@ can be set directly on the :cpp:enumerator:`LV_PART_SCROLLBAR` part of a Widget,
 on the Widget's LV_PART_MAIN part, or that of any of its parents, to make a scrollbar
 inherit the base direction.
 
-``pad_left/right/top/bottom`` sets the spacing around the scrollbars and
+``pad_left/right/top/bottom`` sets the spacing around the scrollbars,
-``width`` sets the scrollbar's width.
+``width`` sets the scrollbar's width and ``length`` sets the scrollbar's length:
+If `length` is not set or left at `0` the scrollbar's length will be set automatically according to the length of the content.  
+.. code-block:: c
 
+   static lv_style_t style_scrollbar;
+   lv_style_init(&style_scrollbar);
+   lv_style_set_pad_left(&style_scrollbar, 2);
+   lv_style_set_pad_right(&style_scrollbar, 2);
+   lv_style_set_pad_top(&style_scrollbar, 2);
+   lv_style_set_pad_bottom(&style_scrollbar, 2);
+   lv_style_set_width(&style_scrollbar, 10);
+   lv_style_set_length(&style_scrollbar, 50);
+
+   ...
+
+   lv_obj_add_style(widget, &style_scrollbar, LV_PART_SCROLLBAR);
+
+The minimum length of the scrollbar is fixed to 10, while its maximum length is limited by the
+Widget's height or width, depending on whether the scrollbar is vertical or horizontal. Any length value
+set outside of these limits will automatically result in a length fixed to either limit.
 
 .. _scroll_events:
 

examples/scroll/lv_example_scroll_4.c

@@ -39,6 +39,7 @@ void lv_example_scroll_4(void)
     static lv_style_t style;
     lv_style_init(&style);
     lv_style_set_width(&style, 4);      /*Width of the scrollbar*/
+    lv_style_set_length(&style, 20);    /*Length of the scrollbar*/
     lv_style_set_pad_right(&style, 5);  /*Space from the parallel side*/
     lv_style_set_pad_top(&style, 5);    /*Space from the perpendicular side*/
 

src/core/lv_obj_scroll.c

@@ -504,6 +504,7 @@ void lv_obj_get_scrollbar_area(lv_obj_t * obj, lv_area_t * hor_area, lv_area_t *
     int32_t left_space = lv_obj_get_style_pad_left(obj, LV_PART_SCROLLBAR);
     int32_t right_space = lv_obj_get_style_pad_right(obj, LV_PART_SCROLLBAR);
     int32_t thickness = lv_obj_get_style_width(obj, LV_PART_SCROLLBAR);
+    int32_t length = lv_obj_get_style_length(obj, LV_PART_SCROLLBAR);
 
     int32_t obj_h = lv_obj_get_height(obj);
     int32_t obj_w = lv_obj_get_width(obj);
@@ -533,7 +534,8 @@ void lv_obj_get_scrollbar_area(lv_obj_t * obj, lv_area_t * hor_area, lv_area_t *
         }
 
         int32_t sb_h = ((obj_h - top_space - bottom_space - hor_req_space) * obj_h) / content_h;
-        sb_h = LV_MAX(sb_h, SCROLLBAR_MIN_SIZE);
+        sb_h = LV_MAX(length > 0 ? length : sb_h, SCROLLBAR_MIN_SIZE); /*Style-defined size, calculated size, or minimum size*/
+        sb_h = LV_MIN(sb_h, obj_h); /*Limit scrollbar length to parent height*/
         rem = (obj_h - top_space - bottom_space - hor_req_space) -
               sb_h;  /*Remaining size from the scrollbar track that is not the scrollbar itself*/
         int32_t scroll_h = content_h - obj_h; /*The size of the content which can be really scrolled*/
@@ -571,7 +573,8 @@ void lv_obj_get_scrollbar_area(lv_obj_t * obj, lv_area_t * hor_area, lv_area_t *
         hor_area->x2 = obj->coords.x2;
 
         int32_t sb_w = ((obj_w - left_space - right_space - ver_reg_space) * obj_w) / content_w;
-        sb_w = LV_MAX(sb_w, SCROLLBAR_MIN_SIZE);
+        sb_w = LV_MAX(length > 0 ? length : sb_w, SCROLLBAR_MIN_SIZE); /*Style-defined size, calculated size, or minimum size*/
+        sb_w = LV_MIN(sb_w, obj_w); /*Limit scrollbar length to parent width*/
         rem = (obj_w - left_space - right_space - ver_reg_space) -
               sb_w;  /*Remaining size from the scrollbar track that is not the scrollbar itself*/
         int32_t scroll_w = content_w - obj_w; /*The size of the content which can be really scrolled*/