diff --git a/AGENTS.md b/AGENTS.md
index 3acf6ee..1680e5d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -47,7 +47,7 @@ Ely GPUI Component. A component library for GPUI, in light and dark.
 
 - Colors, control and icon sizes, text sizes, radii, shadows come from `cx.theme()`. No raw `px()` literals inside components.
 - Gaps and padding use gpui's rem scale helpers (`gap_2`, `px_3`). That scale is the spacing system.
-- Borders use gpui's fixed-pixel helpers (`border_1` is 1px). Hairlines do not scale.
+- Borders use gpui's fixed-pixel helpers (`border_1` is 1px). Hairlines do not scale. gpui paints a border no wider than half its box, so a line as thin as its box is a fill, `w_px` or `h_px` with `bg`, as the resize handles draw theirs.
 - One component, one home. Duplicates point to it.
 - Fail fast. No silent fallbacks. Log state changes with `log`.
 - A handler a component needs is an argument of `new`, so leaving it out fails to compile. A state live data or the outside world can reach, a key gone from a list that changed or a failed load, logs an error naming the owner and draws a visible state, nothing chosen or an error line, and the app runs on. An assert on a caller's argument or on a fact true by construction stays; one that runtime data or input can trip is fixed. Chapters move to this as `tasks/panics.md` ticks them.
diff --git a/src/layout/split.rs b/src/layout/split.rs
index 917a137..0a15585 100644
--- a/src/layout/split.rs
+++ b/src/layout/split.rs
@@ -29,6 +29,7 @@ fn handle(id: impl Into<ElementId>, axis: Axis, line: Hsla, cx: &App) -> Statefu
         .flex()
         .justify_center()
         .items_center();
+    // A filled line: gpui paints a border no wider than half its box.
     match axis {
         Axis::Horizontal => base
             .w(hit)
@@ -36,10 +37,10 @@ fn handle(id: impl Into<ElementId>, axis: Axis, line: Hsla, cx: &App) -> Statefu
             .cursor(CursorStyle::ResizeLeftRight)
             .child(
                 div()
+                    .w_px()
                     .h_full()
-                    .border_l_1()
-                    .border_color(line)
-                    .group_hover("resize", |s| s.border_color(lit)),
+                    .bg(line)
+                    .group_hover("resize", |s| s.bg(lit)),
             ),
         Axis::Vertical => base
             .h(hit)
@@ -47,10 +48,10 @@ fn handle(id: impl Into<ElementId>, axis: Axis, line: Hsla, cx: &App) -> Statefu
             .cursor(CursorStyle::ResizeUpDown)
             .child(
                 div()
+                    .h_px()
                     .w_full()
-                    .border_t_1()
-                    .border_color(line)
-                    .group_hover("resize", |s| s.border_color(lit)),
+                    .bg(line)
+                    .group_hover("resize", |s| s.bg(lit)),
             ),
     }
 }