From fc77f4cc7f4dac2637c5ed13ebb75a66c8f85e30 Mon Sep 17 00:00:00 2001
From: skylar taylor-barrick <hi@skylar.cc>
Date: Sat, 10 Oct 2026 09:40:28 -0600
Subject: [PATCH] UI.7: preserve split resizing beside panel scrollbars

---
 crates/ui/src/docking/surface.rs | 14 ++++----
 crates/ui/src/docking/tests.rs   | 59 ++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+), 6 deletions(-)

diff --git a/crates/ui/src/docking/surface.rs b/crates/ui/src/docking/surface.rs
index cc4e7b05..d993ffa5 100644
--- a/crates/ui/src/docking/surface.rs
+++ b/crates/ui/src/docking/surface.rs
@@ -218,6 +218,14 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
                 let (lo, hi) = split_bounds(wanted.available(), *axis, self.node_limits(first), self.node_limits(second));
                 let first_size = wanted.first_size().clamp(lo, hi);
                 let Some(split) = split_rect(rect, *axis, SplitSize::FixedFirst(first_size), self.style.gap, 0.0) else { return };
+                path.push(false);
+                self.node(ui, first, split.first, path);
+                path.pop();
+                path.push(true);
+                self.node(ui, second, split.second, path);
+                path.pop();
+                // The divider's inner-edge hit margin must win over child scrollbars, just
+                // as accordion dividers win over the adjacent panel's content.
                 let response = ResizeHandle::new(
                     self.id.with(("split", &*path)),
                     *axis,
@@ -232,12 +240,6 @@ impl<P: Clone + Eq + Hash + Debug + Send + Sync + 'static> Renderer<'_, P> {
                 {
                     self.output.actions.push(Action::ResizeSplit { path: path.clone(), size });
                 }
-                path.push(false);
-                self.node(ui, first, split.first, path);
-                path.pop();
-                path.push(true);
-                self.node(ui, second, split.second, path);
-                path.pop();
             }
             Node::Tabs { panels, active } => self.group(ui, panels, *active, rect, false),
             Node::Stack { entries } => {
diff --git a/crates/ui/src/docking/tests.rs b/crates/ui/src/docking/tests.rs
index efc02b7f..91e374d5 100644
--- a/crates/ui/src/docking/tests.rs
+++ b/crates/ui/src/docking/tests.rs
@@ -509,6 +509,65 @@ fn stack_dividers_resize_adjacent_open_bodies_by_pointer_and_keyboard() {
     assert!((entries[0].height.unwrap() + entries[2].height.unwrap() - 300.0).abs() < 0.001);
 }
 
+#[test]
+fn split_inner_edge_remains_draggable_beside_a_scrollbar() {
+    let ctx = Context::default();
+    let mut layout = Layout {
+        root: Some(Node::Split {
+            axis: SplitAxis::Horizontal,
+            size: SplitSize::FixedFirst(220.0),
+            first: Box::new(tabs(&[1], 0)),
+            second: Box::new(tabs(&[2], 0)),
+        }),
+        floating: Vec::new(),
+    };
+    let frame = |layout: &mut Layout<u32>, events| {
+        let mut output = DockOutput::default();
+        let mut sidebar = Rect::NOTHING;
+        ctx.run_ui(RawInput { screen_rect: Some(Rect::from_min_size(pos2(0.0, 0.0), vec2(800.0, 600.0))), events, ..Default::default() }, |ui| {
+            output = DockArea::new(Id::new("scrolling-dock")).show(
+                ui,
+                layout,
+                &DockStyle::from_ui(ui),
+                |p| format!("Panel {p}"),
+                |_| Permissions::default(),
+                |ui, panel| {
+                    if *panel == 1 {
+                        sidebar = ui.max_rect();
+                        egui::ScrollArea::vertical()
+                            .auto_shrink([false, false])
+                            .scroll_bar_visibility(egui::scroll_area::ScrollBarVisibility::AlwaysVisible)
+                            .show(ui, |ui| {
+                                for row in 0..100 {
+                                    ui.label(format!("Library item {row}"));
+                                }
+                            });
+                    } else {
+                        ui.label("Document");
+                    }
+                },
+            );
+        })
+        .textures_delta
+        .clear();
+        assert!(output.error.is_none());
+        for action in output.actions {
+            layout.apply(action).unwrap();
+        }
+        sidebar
+    };
+    for _ in 0..3 {
+        frame(&mut layout, Vec::new());
+    }
+    let sidebar = frame(&mut layout, Vec::new());
+    let start = pos2(sidebar.right() - 2.0, sidebar.center().y);
+    frame(&mut layout, vec![Event::PointerMoved(start), pointer_button(start, true)]);
+    frame(&mut layout, vec![Event::PointerMoved(start + vec2(40.0, 0.0))]);
+    frame(&mut layout, vec![pointer_button(start + vec2(40.0, 0.0), false)]);
+    let Node::Split { size: SplitSize::FixedFirst(width), .. } = layout.root.unwrap() else { panic!() };
+    assert!((width - 260.0).abs() < 0.001, "sidebar width after inner-edge drag: {width}");
+}
+
 #[test]
 fn restoring_a_panel_beside_a_nested_subtree_preserves_the_complete_workspace() {
     let mut layout = Layout {