Fix editor startup gray flash

editor/src/Platform/Win32EditorHost.h

@@ -96,15 +96,24 @@ inline int RunEditor(HINSTANCE hInstance, int nCmdShow) {
         SendMessageW(hwnd, WM_SETICON, ICON_SMALL, reinterpret_cast<LPARAM>(wc.hIconSm));
     }
 
-    ShowWindow(hwnd, nCmdShow);
-    UpdateWindow(hwnd);
-
     if (!Application::Get().Initialize(hwnd)) {
         DestroyWindow(hwnd);
         UnregisterClassW(wc.lpszClassName, wc.hInstance);
         return 1;
     }
 
+    // Warm up a few hidden frames first. On D3D12 flip-model swap chains the first visible
+    // presents can otherwise show only the host clear color while the UI/render backend settles.
+    constexpr int kBootstrapFrameCount = 3;
+    for (int frameIndex = 0; frameIndex < kBootstrapFrameCount; ++frameIndex) {
+        Application::Get().Render();
+    }
+
+    // Show the window only after the swap chain already contains composed editor frames.
+    ShowWindow(hwnd, nCmdShow);
+    Application::Get().Render();
+    UpdateWindow(hwnd);
+
     MSG msg = {};
     while (msg.message != WM_QUIT) {
         if (PeekMessageW(&msg, nullptr, 0U, 0U, PM_REMOVE)) {