Move preview renderer and HDR pickers above the shader ball

Drop the 'Shader Ball' label; the renderer switch and HDR environment
dropdowns sit side by side at the top of the preview column, above the
swatch. Both pickers take an optional item width.

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

src/ui/MaterialEditorPanel.cpp

@@ -287,8 +287,12 @@ void MaterialEditorPanel::Render() {
 }
 
 void MaterialEditorPanel::RenderPreviewPanel() {
-    ImGui::TextUnformatted("Shader Ball");
-    ImGui::Separator();
+    // Renderer switch and HDR environment side by side above the preview.
+    const float halfWidth =
+        (ImGui::GetContentRegionAvail().x - ImGui::GetStyle().ItemSpacing.x) * 0.5f;
+    m_preview.RenderRendererDropdown(halfWidth);
+    ImGui::SameLine();
+    m_preview.RenderLightingDropdown(halfWidth);
 
     if (m_stage && !m_materialPath.IsEmpty()) {
         // Hypershade-style: with a node selected, the ball previews that
@@ -324,10 +328,6 @@ void MaterialEditorPanel::RenderPreviewPanel() {
         ImGui::Dummy(ImVec2(size, size));
     }
 
-    ImGui::Separator();
-    m_preview.RenderRendererDropdown();
-    m_preview.RenderLightingDropdown();
-
     ImGui::Separator();
     ImGui::TextUnformatted("Node Properties");
     ImGui::Separator();

src/ui/MaterialPreviewRenderer.cpp

@@ -257,9 +257,9 @@ void MaterialPreviewRenderer::UpdateDomeOrientation() {
     m_dirty = true;
 }
 
-void MaterialPreviewRenderer::RenderLightingDropdown() {
+void MaterialPreviewRenderer::RenderLightingDropdown(float width) {
     if (!m_initialized) return;
-    ImGui::SetNextItemWidth(-1.0f);
+    ImGui::SetNextItemWidth(width);
     if (ImGui::BeginCombo("##PreviewLighting", kLightPresets[m_lightPreset].label)) {
         for (int i = 0; i < kLightPresetCount; ++i) {
             if (ImGui::Selectable(kLightPresets[i].label, i == m_lightPreset) && i != m_lightPreset)
@@ -309,13 +309,13 @@ void MaterialPreviewRenderer::SetColorCorrection(int ccMode, const std::string&
     m_dirty = true;
 }
 
-void MaterialPreviewRenderer::RenderRendererDropdown() {
+void MaterialPreviewRenderer::RenderRendererDropdown(float width) {
     pxr::TfToken currentId = m_renderer.GetCurrentRendererId();
     std::string displayName = currentId.IsEmpty()
         ? "Renderer"
         : UsdSceneRenderer::GetRendererDisplayName(currentId);
 
-    ImGui::Button(displayName.c_str(), ImVec2(-1.0f, 0.0f));
+    ImGui::Button(displayName.c_str(), ImVec2(width, 0.0f));
     if (ImGui::IsItemHovered())
         ImGui::SetTooltip("Shader-ball render delegate");
 

src/ui/MaterialPreviewRenderer.h

@@ -50,12 +50,14 @@ public:
                             const std::string& ocioLook);
 
     /// Renderer-delegate picker UI (matches the viewport's dropdown pattern).
-    void RenderRendererDropdown();
+    /// width: ImGui item width (-1 = fill available).
+    void RenderRendererDropdown(float width = -1.0f);
 
     /// HDR-environment picker (External / Room / Interior / Sunset). Presets
     /// map to bundled resources/hdri/*.exr dome-light textures; a preset
     /// whose file is missing falls back to the distant key light.
-    void RenderLightingDropdown();
+    /// width: ImGui item width (-1 = fill available).
+    void RenderLightingDropdown(float width = -1.0f);
 
 private:
     void EnsureInitialized();