Don't reset the viewport unless you have to.

src/render/direct3d/SDL_render_d3d.c

@@ -704,7 +704,6 @@ static int
 D3D_RenderClear(SDL_Renderer * renderer)
 {
     D3D_RenderData *data = (D3D_RenderData *) renderer->driverdata;
-    D3DVIEWPORT9 viewport;
     DWORD color;
     HRESULT result;
 
@@ -712,6 +711,16 @@ D3D_RenderClear(SDL_Renderer * renderer)
         return -1;
     }
 
+    color = D3DCOLOR_ARGB(renderer->a, renderer->r, renderer->g, renderer->b);
+
+    /* Don't reset the viewport if we don't have to! */
+    if (!renderer->viewport.x && !renderer->viewport.y &&
+        renderer->viewport.w == data->pparams.BackBufferWidth &&
+        renderer->viewport.h == data->pparams.BackBufferHeight) {
+        result = IDirect3DDevice9_Clear(data->device, 0, NULL, D3DCLEAR_TARGET, color, 0.0f, 0);
+    } else {
+        D3DVIEWPORT9 viewport;
+
         /* Clear is defined to clear the entire render target */
         viewport.X = 0;
         viewport.Y = 0;
@@ -721,8 +730,6 @@ D3D_RenderClear(SDL_Renderer * renderer)
         viewport.MaxZ = 1.0f;
         IDirect3DDevice9_SetViewport(data->device, &viewport);
 
-    color = D3DCOLOR_ARGB(renderer->a, renderer->r, renderer->g, renderer->b);
-
         result = IDirect3DDevice9_Clear(data->device, 0, NULL, D3DCLEAR_TARGET, color, 0.0f, 0);
 
         /* Reset the viewport */
@@ -733,6 +740,7 @@ D3D_RenderClear(SDL_Renderer * renderer)
         viewport.MinZ = 0.0f;
         viewport.MaxZ = 1.0f;
         IDirect3DDevice9_SetViewport(data->device, &viewport);
+    }
 
     if (FAILED(result)) {
         D3D_SetError("Clear()", result);