Fix bevy_ui_widgets scrollbar bug where scrollbar_size wasn't taken into account (#21835)

# Objective

Bevy ui widget scrollbar implementation didn't correctly handle
scrollbar width inserted by taffy.

## Solution

Correctly handle scrollbar width similarly as it is done in bevy engine
scroll position calculation:


https://github.com/bevyengine/bevy/blob/0d46518eb21df54ba951f503930501bc2639e2a2/crates/bevy_ui/src/layout/mod.rs#L334-L336

## Testing

- In personal project added scrollareas with scrollbars that
automatically attach to overflow: scroll elements and utilize space
provided by taffy scrollbar-width.

Co-authored-by: Alice Cecile <[email protected]>

Author: PPakalns

crates/bevy_ui_widgets/src/scrollbar.rs

@@ -130,7 +130,8 @@ fn scrollbar_on_pointer_down(
         // Convert the click coordinates into a scroll position. If it's greater than the
         // current scroll position, scroll forward by one step (visible size) otherwise scroll
         // back.
-        let visible_size = scroll_content.size() * scroll_content.inverse_scale_factor;
+        let visible_size = (scroll_content.size() - scroll_content.scrollbar_size)
+            * scroll_content.inverse_scale_factor;
         let content_size = scroll_content.content_size() * scroll_content.inverse_scale_factor;
         let max_range = (content_size - visible_size).max(Vec2::ZERO);
 
@@ -193,8 +194,11 @@ fn scrollbar_on_drag(
 
         if drag.dragging {
             let distance = ev.event().distance / ui_scale.0;
-            let visible_size = scroll_content.size() * scroll_content.inverse_scale_factor;
+
+            let visible_size = (scroll_content.size() - scroll_content.scrollbar_size)
+                * scroll_content.inverse_scale_factor;
             let content_size = scroll_content.content_size() * scroll_content.inverse_scale_factor;
+
             let scrollbar_size = (node.size() * node.inverse_scale_factor).max(Vec2::ONE);
 
             match scrollbar.orientation {
@@ -250,7 +254,8 @@ fn update_scrollbar_thumb(
         };
 
         // Size of the visible scrolling area.
-        let visible_size = scroll_area.1.size() * scroll_area.1.inverse_scale_factor;
+        let visible_size = (scroll_area.1.size() - scroll_area.1.scrollbar_size)
+            * scroll_area.1.inverse_scale_factor;
 
         // Size of the scrolling content.
         let content_size = scroll_area.1.content_size() * scroll_area.1.inverse_scale_factor;