fix: exclude scroll area padding from pane sizing

crates/core/tests/scroll_area.rs

@@ -1,7 +1,7 @@
 use gpui::{
-    Context, IntoElement, Modifiers, ParentElement as _, Render, ScrollDelta, ScrollEvent,
-    ScrollHandle, SharedString, Styled as _, TestAppContext, VisualTestContext, Window, div, point,
-    px,
+    Context, InteractiveElement as _, IntoElement, Modifiers, ParentElement as _, Render,
+    ScrollDelta, ScrollEvent, ScrollHandle, SharedString, Styled as _, TestAppContext,
+    VisualTestContext, Window, div, point, px,
 };
 use gpui_native_core::scroll_area::ScrollArea;
 use gpui_native_theme::Theme;
@@ -116,6 +116,53 @@ impl Render for GalleryShell {
     }
 }
 
+/// A window-filling scroll area with `p_4`, over one full-width row.
+struct PaddedShell {
+    scroll: ScrollHandle,
+}
+
+impl Render for PaddedShell {
+    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
+        div()
+            .size_full()
+            .debug_selector(|| "padded-root".to_string())
+            .child(
+                ScrollArea::new("padded-scroll")
+                    .handle(self.scroll.clone())
+                    .size_full()
+                    .p_4()
+                    .child(
+                        div()
+                            .debug_selector(|| "padded-row".to_string())
+                            .h(px(20.))
+                            .w_full(),
+                    ),
+            )
+    }
+}
+
+#[test]
+fn padding_insets_the_content_exactly_once() {
+    let mut cx = TestAppContext::single();
+    cx.update(Theme::init);
+    let handle = ScrollHandle::new();
+    let handle_for_view = handle.clone();
+    let window = cx.add_window(move |_, _| PaddedShell {
+        scroll: handle_for_view,
+    });
+    let mut visual = VisualTestContext::from_window(window.into(), &cx);
+    visual.update(|window, cx| {
+        window.draw(cx).clear(cx);
+    });
+
+    let root = visual.debug_bounds("padded-root").expect("root rendered");
+    let row = visual.debug_bounds("padded-row").expect("row rendered");
+    // p_4 = 16px: the padding reaches only the wrapper, so the content is
+    // inset once, not once per wrapper and scroll pane.
+    assert_eq!(row.origin.x - root.origin.x, px(16.));
+    assert_eq!(root.size.width - row.size.width, px(32.));
+}
+
 #[test]
 fn a_gallery_style_window_fill_scroll_area_scrolls_tall_flex_content() {
     let mut cx = TestAppContext::single();