fix: avoid editor viewport resize deadlock

editor/src/Platform/D3D12WindowRenderer.h

@@ -155,6 +155,23 @@ public:
         RecreateBackBufferViews();
     }
 
+    bool BeginFrame() {
+        auto* d3d12Queue = GetD3D12CommandQueue();
+        auto* d3d12CommandList = GetD3D12CommandList();
+        if (m_swapChain == nullptr ||
+            d3d12Queue == nullptr ||
+            d3d12CommandList == nullptr ||
+            m_srvHeap == nullptr) {
+            return false;
+        }
+
+        // Viewport panels can resize and recreate render targets while building ImGui.
+        // Make sure the previous frame is fully retired before any CPU-side resource destruction.
+        d3d12Queue->WaitForPreviousFrame();
+        m_commandList->Reset();
+        return true;
+    }
+
     void Render(
         UI::ImGuiBackendBridge& imguiBackend,
         const float clearColor[4],
@@ -168,9 +185,6 @@ public:
             return;
         }
 
-        d3d12Queue->WaitForPreviousFrame();
-        m_commandList->Reset();
-
         if (beforeUiRender) {
             beforeUiRender(GetRenderContext());
         }