editor: Do not insert scrollbar hitboxes when scrollbars are never to be shown (#29316)

This PR fixes an issue where scrollbar hitboxes were still inserted for
editors despite scrollbars being programmatically disabled via the
`show_scrollbars`field. This is basically the same fix as in #27467.

The thought process here is that the motivation for `show_scrollbars` is
not to just hide the scrollbars in the editor, but to fully disable
scrollbars for the associated editor. However, this is currently not the
case, as a functioning hitbox for each scrollbar is stil inserted. For
example, the behavior with the old assistant panel can be seen below:


https://github.com/user-attachments/assets/18af6338-dd28-4794-a6a6-5b9691b243f2

Whilst the scrollbar is not visible, there is still a scrollbar hitbox
inserted which triggers hover events and is fully functioning.


This PR fixes this by fully skipping the scrollbar layouting whenever
`show_scrollbars` is set to false, preventing the hitboxes from being
inserted.


https://github.com/user-attachments/assets/b6bb6dc7-902f-4383-bf03-506d0a57ec77


Release Notes:

- N/A

crates/editor/src/element.rs

@@ -1449,7 +1449,7 @@ impl EditorElement {
         window: &mut Window,
         cx: &mut App,
     ) -> Option<EditorScrollbars> {
-        if !snapshot.mode.is_full() {
+        if !snapshot.mode.is_full() || !self.editor.read(cx).show_scrollbars {
             return None;
         }
 
@@ -1462,8 +1462,7 @@ impl EditorElement {
         }
 
         let scrollbar_settings = EditorSettings::get_global(cx).scrollbar;
-        let show_scrollbars = self.editor.read(cx).show_scrollbars
-            && match scrollbar_settings.show {
+        let show_scrollbars = match scrollbar_settings.show {
             ShowScrollbar::Auto => {
                 let editor = self.editor.read(cx);
                 let is_singleton = editor.is_singleton(cx);