gpui: Round scroll_max to two decimal places (#34832)

Follow up to #31836 

After enabling rounding in the Taffy layout engine, we frequently run
into cases where the bounds produced by Taffy and ours slightly differ
after 5 or more decimal places. This leads to cases where containers
become scrollable for less than 0.0000x Pixels. In case this happens for
e.g. hover popovers, we render a scrollbar due to the container being
technically scrollable, even though the scroll amount here will in
practice never be visible.

This change fixes this by rounding the `scroll_max` by which we clamp
the current scroll position to two decimal places. We don't benefit from
the additional floating point precision here at all and it stops such
containers from becoming scrollable altogether. Furthermore, we now
store the `scroll_max` instead of the `padded_content_size` as the
former gives a much better idea on whether the corresponding container
is scrollable or not.

| `main` | After these changes |
| -- | -- |
| <img width="610" height="316" alt="main"
src="https://github.com/user-attachments/assets/ffcc0322-6d6e-4f79-a916-bd3c57fe4211"
/> | <img width="610" height="316" alt="scroll_max_rounded"
src="https://github.com/user-attachments/assets/5fe530f5-2e21-4aaa-81f4-e5c53ab73e4f"
/> |

Release Notes:

- Fixed an issue where scrollbars would appear in containers where no
scrolling was possible.

crates/gpui/src/elements/list.rs

@@ -411,9 +411,9 @@ impl ListState {
         self.0.borrow_mut().set_offset_from_scrollbar(point);
     }
 
-    /// Returns the size of items we have measured.
+    /// Returns the maximum scroll offset according to the items we have measured.
     /// This value remains constant while dragging to prevent the scrollbar from moving away unexpectedly.
-    pub fn content_size_for_scrollbar(&self) -> Size<Pixels> {
+    pub fn max_offset_for_scrollbar(&self) -> Size<Pixels> {
         let state = self.0.borrow();
         let bounds = state.last_layout_bounds.unwrap_or_default();
 
@@ -421,7 +421,7 @@ impl ListState {
             .scrollbar_drag_start_height
             .unwrap_or_else(|| state.items.summary().height);
 
-        Size::new(bounds.size.width, height)
+        Size::new(Pixels::ZERO, Pixels::ZERO.max(height - bounds.size.height))
     }
 
     /// Returns the current scroll offset adjusted for the scrollbar