feat(paint-slots): paint panel polish — sticky controls, selection outlines, auto-select

- MaterialPaintPanel owns its scroll: the eraser/reset row stays pinned and
  the category tabs stick to the top, so only the material list scrolls.
- Selected catalog swatch + active scene-material card use the same
  `ring-1 ring-primary ring-inset` outline as item/preset tiles.
- Choosing a material category auto-selects its first material.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/editor/src/components/ui/controls/material-paint-panel.tsx

@@ -56,8 +56,11 @@ export function MaterialPaintPanel() {
   }
 
   return (
-    <div className="w-full space-y-2">
-      <div className="flex items-center gap-2">
+    // Fill the host's scroll slot and own the scroll internally: the eraser /
+    // reset row stays pinned (shrink-0) while only the material list below
+    // scrolls. The category tabs pin too (sticky, inside the scroll region).
+    <div className="flex h-full min-h-0 w-full flex-col">
+      <div className="flex shrink-0 items-center gap-2 pb-2">
         <Button
           aria-pressed={paintEraser}
           className="flex-1"
@@ -79,32 +82,34 @@ export function MaterialPaintPanel() {
           Reset all
         </Button>
       </div>
-      <MaterialPicker
-        onChange={(material) => {
-          // Custom-create: pre-create a scene material and select it as the
-          // brush via a `scene:` ref so painting stores the ref and edits to
-          // it propagate everywhere. The user edits it inline in the scene-
-          // material list below (auto-opened) — no separate right-side pane.
-          const id = generateSceneMaterialId()
-          const count = Object.keys(useScene.getState().materials).length
-          useScene.getState().addSceneMaterial({ id, name: `Material ${count + 1}`, material })
-          setActivePaintMaterial({
-            materialPreset: toSceneMaterialRef(id),
-            sourceTarget: activePaintTarget,
-          })
-          setAutoEditMaterialId(id)
-        }}
-        onSelectMaterialPreset={(materialPreset) => {
-          setActivePaintMaterial({ materialPreset, sourceTarget: activePaintTarget })
-        }}
-        selectedMaterialPreset={activePaintMaterial?.materialPreset}
-        value={activePaintMaterial?.material}
-      />
-      {materialCount > 0 ? (
-        <PanelSection title="Scene materials">
-          <SceneMaterialList autoEditId={autoEditMaterialId} />
-        </PanelSection>
-      ) : null}
+      <div className="subtle-scrollbar min-h-0 flex-1 space-y-2 overflow-y-auto">
+        <MaterialPicker
+          onChange={(material) => {
+            // Custom-create: pre-create a scene material and select it as the
+            // brush via a `scene:` ref so painting stores the ref and edits to
+            // it propagate everywhere. The user edits it inline in the scene-
+            // material list below (auto-opened) — no separate right-side pane.
+            const id = generateSceneMaterialId()
+            const count = Object.keys(useScene.getState().materials).length
+            useScene.getState().addSceneMaterial({ id, name: `Material ${count + 1}`, material })
+            setActivePaintMaterial({
+              materialPreset: toSceneMaterialRef(id),
+              sourceTarget: activePaintTarget,
+            })
+            setAutoEditMaterialId(id)
+          }}
+          onSelectMaterialPreset={(materialPreset) => {
+            setActivePaintMaterial({ materialPreset, sourceTarget: activePaintTarget })
+          }}
+          selectedMaterialPreset={activePaintMaterial?.materialPreset}
+          value={activePaintMaterial?.material}
+        />
+        {materialCount > 0 ? (
+          <PanelSection title="Scene materials">
+            <SceneMaterialList autoEditId={autoEditMaterialId} />
+          </PanelSection>
+        ) : null}
+      </div>
     </div>
   )
 }

packages/editor/src/components/ui/controls/material-picker.tsx

@@ -96,7 +96,7 @@ export function MaterialPicker({
     <div className={`min-w-0 space-y-3 ${disabled ? 'pointer-events-none opacity-50' : ''}`}>
       {(catalogItems.length > 0 || onChange) && (
         <div className="min-w-0 space-y-1">
-          <div className="flex flex-wrap gap-1 pb-1">
+          <div className="sticky top-0 z-10 flex flex-wrap gap-1 bg-sidebar pb-1">
             {availableCategories.map((category) => (
               <button
                 className={`rounded-full px-3 py-1 font-medium text-xs transition-colors ${
@@ -107,9 +107,11 @@ export function MaterialPicker({
                 key={category}
                 onClick={() => {
                   setSelectedCategory(category)
-                  if (showCustom) {
-                    setShowCustom(false)
-                  }
+                  // Auto-select the first material in the category so the brush
+                  // is immediately ready (and the row shows as selected).
+                  const first = getMaterialsForCategory(category)[0]
+                  if (first) handleCatalogSelect(first.id)
+                  else if (showCustom) setShowCustom(false)
                 }}
                 type="button"
               >
@@ -126,7 +128,7 @@ export function MaterialPicker({
               return (
                 <button
                   className={`group relative flex flex-col gap-1.5 rounded-xl p-1.5 transition-colors hover:cursor-pointer hover:bg-sidebar-accent ${
-                    isSelected ? 'bg-sidebar-accent ring-2 ring-primary-foreground' : ''
+                    isSelected ? 'bg-sidebar-accent ring-1 ring-primary ring-inset' : ''
                   }`}
                   key={item.id}
                   onClick={() => {
@@ -159,7 +161,7 @@ export function MaterialPicker({
             {selectedCategory === 'colors' && onChange ? (
               <button
                 className={`group relative flex flex-col gap-1.5 rounded-xl p-1.5 transition-colors hover:cursor-pointer hover:bg-sidebar-accent ${
-                  showCustom ? 'bg-sidebar-accent ring-2 ring-primary-foreground' : ''
+                  showCustom ? 'bg-sidebar-accent ring-1 ring-primary ring-inset' : ''
                 }`}
                 onClick={() => {
                   triggerSFX('sfx:menu-click')

packages/editor/src/components/ui/controls/scene-material-list.tsx

@@ -32,6 +32,7 @@ export function SceneMaterialList({ autoEditId }: { autoEditId?: SceneMaterialId
   const updateSceneMaterial = useScene((state) => state.updateSceneMaterial)
   const removeSceneMaterial = useScene((state) => state.removeSceneMaterial)
   const activePaintTarget = useEditor((state) => state.activePaintTarget)
+  const activePaintRef = useEditor((state) => state.activePaintMaterial?.materialPreset)
   const setActivePaintMaterial = useEditor((state) => state.setActivePaintMaterial)
 
   const materialEntries = useMemo(
@@ -71,6 +72,7 @@ export function SceneMaterialList({ autoEditId }: { autoEditId?: SceneMaterialId
           activePaintTarget={activePaintTarget}
           autoEdit={autoEditId === id}
           id={id}
+          isActive={activePaintRef === toSceneMaterialRef(id)}
           key={id}
           removeSceneMaterial={removeSceneMaterial}
           sceneMaterial={sceneMaterial}
@@ -89,6 +91,7 @@ function SceneMaterialRow({
   usageCount,
   activePaintTarget,
   autoEdit,
+  isActive,
   addSceneMaterial,
   updateSceneMaterial,
   removeSceneMaterial,
@@ -99,6 +102,7 @@ function SceneMaterialRow({
   usageCount: number
   activePaintTarget: ReturnType<typeof useEditor.getState>['activePaintTarget']
   autoEdit: boolean
+  isActive: boolean
   addSceneMaterial: ReturnType<typeof useScene.getState>['addSceneMaterial']
   updateSceneMaterial: ReturnType<typeof useScene.getState>['updateSceneMaterial']
   removeSceneMaterial: ReturnType<typeof useScene.getState>['removeSceneMaterial']
@@ -133,7 +137,11 @@ function SceneMaterialRow({
   }
 
   return (
-    <div className="rounded-md border border-border/60 bg-background/40 p-2">
+    <div
+      className={`rounded-md border border-border/60 bg-background/40 p-2 ${
+        isActive ? 'ring-1 ring-primary ring-inset' : ''
+      }`}
+    >
       <div className="flex items-center gap-2">
         <span
           className="h-8 w-8 shrink-0 rounded-md border border-border/70"