Zip the layer's lists against their pipelines, and stop explaining wgpu wrongly

The update loop zips all three and asserts up front that no list is left
without a pipeline, instead of indexing the pipelines to get that guarantee.

The comment on the data layout claimed a `None` minimum takes its value from
the first pipeline built against the layout. That is not documented and does
not reproduce -- one shared layout with `None` renders the tabs example
correctly today. What is documented is that a stated size is checked when the
bind group and pipeline are created, and `None` is checked on every draw, so
that is what the comment says now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

core/src/render/mod.rs

@@ -117,24 +117,22 @@ impl UiRenderNode {
             }
             let rlayer = self.layers.entry(i).or_insert_with(RenderLayer::new);
             if draws.updated {
-                rlayer
+                let lists = draws.primitives();
-                    .primitives
+                // The zip would otherwise skip a list with no pipeline.
-                    .resize_with(draws.primitives().len(), || None);
+                assert!(lists.len() <= self.primitives.len());
-                for (id, (buffers, list)) in rlayer
+                rlayer.primitives.resize_with(lists.len(), || None);
+                for ((buffers, list), primitive) in rlayer
                     .primitives
                     .iter_mut()
-                    .zip(draws.primitives())
+                    .zip(lists)
-                    .enumerate()
+                    .zip(&self.primitives)
                 {
                     let Some(list) = list else {
                         continue;
                     };
-                    // Indexed, not zipped: a missing pipeline should say so
-                    // rather than quietly leave the list unbuilt.
-                    let layout = &self.primitives[id].data_layout;
                     buffers
                         .get_or_insert_with(|| ListBuffers::new(device))
-                        .update(device, queue, layout, list);
+                        .update(device, queue, &primitive.data_layout, list);
                 }
                 draws.updated = false;
             }
@@ -364,9 +362,8 @@ impl UiRenderNode {
         })
     }
 
-    /// One list's per-instance data. Each primitive needs its own, stating its
+    /// Layout for a list of one primitive's data. Stating the size rather than
-    /// own `stride`: a `None` minimum takes its value from the first pipeline
+    /// leaving it `None` is what moves the check off every draw.
-    /// built against the layout, and then holds every later one to that.
     fn data_layout(device: &Device, stride: u64) -> BindGroupLayout {
         device.create_bind_group_layout(&BindGroupLayoutDescriptor {
             entries: &[BindGroupLayoutEntry {
@@ -442,7 +439,6 @@ impl ListBuffers {
         self.slots.extend_from_slice(list.slots());
         self.instance.update(device, queue, list.instances());
         let resized = self.data.update(device, queue, list.data());
-        // An empty list has no buffer big enough to bind, and nothing to draw.
         if list.instances().is_empty() {
             self.group = None;
         } else if resized || self.group.is_none() {

core/src/ui/painter.rs

@@ -101,6 +101,7 @@ impl<'a> Painter<'a> {
         ui.text.render(buffer, attrs, width)
     }
 
+    // TODO: merge the text methods into the primitive ones.
     pub fn glyphs(&mut self, text: &RenderedText, origin: UiRegion) {
         let kind = self.rsc.ui_mut().primitives.kind::<GlyphPrimitive>();
         for glyph in text.glyphs.iter() {