diff --git a/core/src/render/mod.rs b/core/src/render/mod.rs
index d368ffb..834cc8c 100644
--- a/core/src/render/mod.rs
+++ b/core/src/render/mod.rs
@@ -22,6 +22,10 @@ pub use primitive::*;
 
 const PRELUDE: &str = include_str!("./shader/prelude.wgsl");
 
+fn module_source(wgsl: &str) -> String {
+    format!("{PRELUDE}\n{wgsl}")
+}
+
 pub struct UiRenderNode {
     shared_layout: BindGroupLayout,
     shared_group: BindGroup,
@@ -222,7 +226,7 @@ impl UiRenderNode {
     ) -> RenderPipeline {
         let module = device.create_shader_module(ShaderModuleDescriptor {
             label: Some(label),
-            source: ShaderSource::Wgsl(format!("{PRELUDE}\n{wgsl}").into()),
+            source: ShaderSource::Wgsl(module_source(wgsl).into()),
         });
         device.create_render_pipeline(&RenderPipelineDescriptor {
             label: Some(label),
@@ -401,3 +405,35 @@ impl ListBuffers {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::module_source;
+    use wgpu::naga::{
+        front::wgsl,
+        valid::{Capabilities, ValidationFlags, Validator},
+    };
+
+    /// Every shader file, composed as the renderer composes it, parses and
+    /// validates with no device -- so an edit that breaks one fails here and
+    /// not in the first window opened.
+    #[test]
+    fn every_shader_validates() {
+        let dir = concat!(env!("CARGO_MANIFEST_DIR"), "/src/render/shader");
+        let mut checked = 0;
+        for entry in std::fs::read_dir(dir).unwrap() {
+            let path = entry.unwrap().path();
+            if path.extension().is_none_or(|e| e != "wgsl") || path.ends_with("prelude.wgsl") {
+                continue;
+            }
+            let source = module_source(&std::fs::read_to_string(&path).unwrap());
+            let module = wgsl::parse_str(&source)
+                .unwrap_or_else(|e| panic!("{}: {}", path.display(), e.emit_to_string(&source)));
+            Validator::new(ValidationFlags::all(), Capabilities::all())
+                .validate(&module)
+                .unwrap_or_else(|e| panic!("{}: {e:?}", path.display()));
+            checked += 1;
+        }
+        assert!(checked > 0, "no shaders found in {dir}");
+    }
+}
diff --git a/core/src/render/shader/rect.wgsl b/core/src/render/shader/rect.wgsl
index 5a01564..6d8694d 100644
--- a/core/src/render/shader/rect.wgsl
+++ b/core/src/render/shader/rect.wgsl
@@ -34,6 +34,6 @@ fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner:
     // vec from center to pixel
     let p = pixel_pos - rect_center;
     // vec from inner rect corner to pixel
-    let q = px(p) - (rect_corner - radius);
+    let q = abs(p) - (rect_corner - radius);
     return length(max(q, vec2(0.0))) - radius;
 }