Fix: ContainerView respects frame width constraints

Sources/TUIkit/Views/ContainerView.swift

@@ -357,9 +357,14 @@ private struct _ContainerViewCore<Content: View, Footer: View>: View, Renderable
         }
 
         // Calculate inner width from title, body, AND footer.
+        // If a frame constraint is set (availableWidth is smaller than terminal),
+        // use that as the maximum width.
         let titleWidth = title.map { $0.count + 4 } ?? 0  // " Title " + borders
         let footerNaturalWidth = initialFooterBuffer?.width ?? 0
-        let innerWidth = max(titleWidth, bodyBuffer.width, footerNaturalWidth)
+        let contentBasedWidth = max(titleWidth, bodyBuffer.width, footerNaturalWidth)
+        
+        // Respect frame constraints: innerWidth should not exceed innerContext.availableWidth
+        let innerWidth = min(contentBasedWidth, innerContext.availableWidth)
 
         // Re-render footer constrained to the final innerWidth so that
         // Spacer() fills exactly the container's inner width.

Tests/TUIkitTests/ListTests.swift

@@ -198,4 +198,25 @@ struct ListRenderingTests {
 
         #expect(list.selectionMode == .single)
     }
+
+    @Test("List respects frame width constraint")
+    func listRespectsFrameWidth() {
+        let context = createTestContext(width: 80)
+
+        var selection: String?
+        let list = List("Items", selection: Binding(
+            get: { selection },
+            set: { selection = $0 }
+        )) {
+            ForEach(["Alpha", "Beta", "Gamma"], id: \.self) { item in
+                Text(item)
+            }
+        }
+        .frame(width: 20)
+
+        let buffer = renderToBuffer(list, context: context)
+
+        // The list should be constrained to 20 characters width
+        #expect(buffer.width == 20)
+    }
 }