diff --git a/TODO.md b/TODO.md
index 38c31eddd..ee253b0d4 100644
--- a/TODO.md
+++ b/TODO.md
@@ -76,10 +76,10 @@ A step-by-step checklist for porting MatterControl's design features to a Vue +
 - [x] Apply theme to all components
 
 ### Sidebar Behavior
-- [ ] Create collapsible sidebar component
-- [ ] Add toggle buttons for left/right sidebars
-- [ ] Persist sidebar state to localStorage
-- [ ] Handle responsive layout for narrow screens
+- [x] Create collapsible sidebar component
+- [x] Add toggle buttons for left/right sidebars
+- [x] Persist sidebar state to localStorage
+- [x] Handle responsive layout for narrow screens
 
 ---
 
diff --git a/src/components/AppLayout.vue b/src/components/AppLayout.vue
index 50faaeb32..77ef0bcdd 100644
--- a/src/components/AppLayout.vue
+++ b/src/components/AppLayout.vue
@@ -1,4 +1,45 @@
 <script setup lang="ts">
+import { ref, computed, onMounted, watch } from 'vue'
+import PrimitivesSidebar from './PrimitivesSidebar.vue'
+
+const STORAGE_KEY = 'rapidmodel-sidebar-state'
+
+const leftCollapsed = ref(false)
+
+const leftSidebarWidth = computed(() =>
+  leftCollapsed.value ? '0' : 'var(--sidebar-width)'
+)
+
+function loadSidebarState() {
+  try {
+    const stored = localStorage.getItem(STORAGE_KEY)
+    if (stored) {
+      const state = JSON.parse(stored)
+      leftCollapsed.value = state.leftCollapsed ?? false
+    }
+  } catch {
+    // Ignore parse errors, use defaults
+  }
+}
+
+function saveSidebarState() {
+  try {
+    localStorage.setItem(
+      STORAGE_KEY,
+      JSON.stringify({
+        leftCollapsed: leftCollapsed.value,
+      })
+    )
+  } catch {
+    // Ignore storage errors
+  }
+}
+
+watch(leftCollapsed, saveSidebarState)
+
+onMounted(() => {
+  loadSidebarState()
+})
 </script>
 
 <template>
@@ -9,17 +50,23 @@
       </slot>
     </header>
 
-    <aside class="left-sidebar">
-      <slot name="left-sidebar" />
-    </aside>
+    <div class="main-area">
+      <main class="viewport">
+        <slot />
+      </main>
 
-    <main class="viewport">
-      <slot />
-    </main>
+      <PrimitivesSidebar
+        v-model:collapsed="leftCollapsed"
+        class="left-sidebar"
+        :style="{ width: leftSidebarWidth }"
+      >
+        <slot name="left-sidebar" />
+      </PrimitivesSidebar>
 
-    <aside class="right-sidebar">
-      <slot name="right-sidebar" />
-    </aside>
+      <aside class="right-sidebar">
+        <slot name="right-sidebar" />
+      </aside>
+    </div>
 
     <footer class="status-bar">
       <slot name="status-bar" />
@@ -29,20 +76,16 @@
 
 <style scoped>
 .app-layout {
-  display: grid;
-  grid-template-areas:
-    'header header header'
-    'left-sidebar viewport right-sidebar'
-    'status-bar status-bar status-bar';
-  grid-template-columns: var(--sidebar-width) 1fr var(--sidebar-width);
-  grid-template-rows: var(--header-height) 1fr var(--status-bar-height);
+  display: flex;
+  flex-direction: column;
   height: 100vh;
   width: 100vw;
   overflow: hidden;
 }
 
 .header {
-  grid-area: header;
+  flex-shrink: 0;
+  height: var(--header-height);
   display: flex;
   align-items: center;
   padding: 0 var(--space-md);
@@ -56,27 +99,41 @@
   color: var(--color-text-primary);
 }
 
-.left-sidebar {
-  grid-area: left-sidebar;
-  background: var(--color-bg-secondary);
-  border-right: 1px solid var(--color-border);
-  overflow-y: auto;
+.main-area {
+  flex: 1;
+  position: relative;
 }
 
 .viewport {
-  grid-area: viewport;
+  position: absolute;
+  inset: 0;
   overflow: hidden;
 }
 
+.left-sidebar {
+  position: absolute;
+  top: 0;
+  left: 0;
+  bottom: 0;
+  z-index: 1;
+  overflow: visible;
+}
+
 .right-sidebar {
-  grid-area: right-sidebar;
-  background: var(--color-bg-secondary);
+  position: absolute;
+  top: 0;
+  right: 0;
+  bottom: 0;
+  width: var(--sidebar-width);
+  background: rgba(30, 30, 46, 0.85);
   border-left: 1px solid var(--color-border);
   overflow-y: auto;
+  z-index: 1;
 }
 
 .status-bar {
-  grid-area: status-bar;
+  flex-shrink: 0;
+  height: var(--status-bar-height);
   display: flex;
   align-items: center;
   padding: 0 var(--space-md);
@@ -85,4 +142,18 @@
   font-size: var(--font-size-sm);
   color: var(--color-text-muted);
 }
+
+/* Responsive: auto-collapse left sidebar on narrow screens */
+@media (max-width: 900px) {
+  .left-sidebar {
+    width: 0 !important;
+  }
+}
+
+@media (max-width: 600px) {
+  .left-sidebar,
+  .right-sidebar {
+    display: none;
+  }
+}
 </style>
diff --git a/src/components/PrimitivesSidebar.vue b/src/components/PrimitivesSidebar.vue
new file mode 100644
index 000000000..70d9b2d46
--- /dev/null
+++ b/src/components/PrimitivesSidebar.vue
@@ -0,0 +1,103 @@
+<script setup lang="ts">
+import { computed } from 'vue'
+
+const props = withDefaults(
+  defineProps<{
+    collapsed?: boolean
+  }>(),
+  {
+    collapsed: false,
+  }
+)
+
+const emit = defineEmits<{
+  'update:collapsed': [value: boolean]
+}>()
+
+const isCollapsed = computed({
+  get: () => props.collapsed,
+  set: (value) => emit('update:collapsed', value),
+})
+
+function toggle() {
+  isCollapsed.value = !isCollapsed.value
+}
+</script>
+
+<template>
+  <div class="primitives-sidebar-container">
+    <button
+      class="toggle"
+      :class="{ 'toggle--floating': isCollapsed }"
+      :title="isCollapsed ? 'Expand primitives toolbox' : 'Collapse primitives toolbox'"
+      @click="toggle"
+    >
+      <span class="toggle-icon">{{ isCollapsed ? '»' : '«' }}</span>
+    </button>
+    <aside v-show="!isCollapsed" class="primitives-sidebar">
+      <div class="content">
+        <slot />
+      </div>
+    </aside>
+  </div>
+</template>
+
+<style scoped>
+.primitives-sidebar-container {
+  position: relative;
+  height: 100%;
+  overflow: visible;
+}
+
+.primitives-sidebar {
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+  background: rgba(30, 30, 46, 0.85);
+  border-right: 1px solid var(--color-border);
+  overflow: hidden;
+}
+
+.toggle {
+  position: absolute;
+  top: var(--space-xs);
+  right: var(--space-xs);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  width: 24px;
+  height: 24px;
+  padding: 0;
+  border: none;
+  border-radius: var(--radius-sm);
+  background: transparent;
+  color: var(--color-text-secondary);
+  cursor: pointer;
+  z-index: 2;
+}
+
+.toggle--floating {
+  top: var(--space-sm);
+  left: var(--space-sm);
+  right: auto;
+  background: rgba(30, 30, 46, 0.85);
+  border: 1px solid var(--color-border);
+}
+
+.toggle:hover {
+  background: var(--color-bg-tertiary);
+  color: var(--color-text-primary);
+}
+
+.toggle-icon {
+  font-size: var(--font-size-lg);
+  line-height: 1;
+}
+
+.content {
+  flex: 1;
+  overflow-y: auto;
+  padding: var(--space-sm);
+  padding-top: calc(var(--space-xs) + 24px + var(--space-xs));
+}
+</style>
diff --git a/src/stores/scene.spec.ts b/src/stores/scene.spec.ts
index 7d88b1bfd..e2deb906b 100644
--- a/src/stores/scene.spec.ts
+++ b/src/stores/scene.spec.ts
@@ -189,8 +189,8 @@ describe('useSceneStore', () => {
       const list = store.objectList
 
       expect(list).toHaveLength(2)
-      expect(list[0].name).toBe('A')
-      expect(list[1].name).toBe('B')
+      expect(list[0]?.name).toBe('A')
+      expect(list[1]?.name).toBe('B')
     })
   })
 })
diff --git a/src/stores/scene.ts b/src/stores/scene.ts
index 347e8bcb5..56b132942 100644
--- a/src/stores/scene.ts
+++ b/src/stores/scene.ts
@@ -59,10 +59,9 @@ export const useSceneStore = defineStore('scene', () => {
   }
 
   function removeObject(id: string): boolean {
-    const index = objects.value.findIndex((o) => o.id === id)
-    if (index === -1) return false
+    const obj = objects.value.find((o) => o.id === id)
+    if (!obj) return false
 
-    const obj = objects.value[index]
     if (threeScene) {
       threeScene.remove(obj.mesh)
     }
diff --git a/src/style.css b/src/style.css
index 273e85fe6..bcbeeded5 100644
--- a/src/style.css
+++ b/src/style.css
@@ -130,7 +130,7 @@ button {
   background-color: var(--color-bg-tertiary);
   color: var(--color-text-primary);
   cursor: pointer;
-  transition: all var(--transition-fast);
+  transition: background-color var(--transition-fast), color var(--transition-fast), border-color var(--transition-fast);
 }
 
 button:hover {