Question

Normally, even when using double buffering, when resizing a window, it seems that it’s inevitable that the flickering will happen.

Step 1, the original window.

Step 2, the window is resized, but the extra area hasn’t been painted.

Step 3, the window is resized, and the extra area has been painted.

Is it possible somehow to hide setp 2? Can I suspend the resizing process until the painting action is done?

Here’s an example:

#include <Windows.h>
#include <windowsx.h>
#include <Uxtheme.h>

#pragma comment(lib, "Uxtheme.lib")

LRESULT CALLBACK WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
BOOL MainWindow_OnCreate(HWND hWnd, LPCREATESTRUCT lpCreateStruct);
void MainWindow_OnDestroy(HWND hWnd);
void MainWindow_OnSize(HWND hWnd, UINT state, int cx, int cy);
void MainWindow_OnPaint(HWND hWnd);

int APIENTRY wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPWSTR lpCmdLine, int nCmdShow)
{
  WNDCLASSEX wcex = { 0 };
  HWND hWnd;
  MSG msg;
  BOOL ret;

  wcex.cbSize = sizeof(wcex);
  wcex.lpfnWndProc = WindowProc;
  wcex.hInstance = hInstance;
  wcex.hIcon = (HICON)LoadImage(NULL, IDI_APPLICATION, IMAGE_ICON, 0, 0, LR_SHARED);
  wcex.hCursor = (HCURSOR)LoadImage(NULL, IDC_ARROW, IMAGE_CURSOR, 0, 0, LR_SHARED);
  wcex.lpszClassName = TEXT("MainWindow");
  wcex.hIconSm = wcex.hIcon;

  if (!RegisterClassEx(&wcex))
  {
    return 1;
  }

  hWnd = CreateWindow(wcex.lpszClassName, TEXT("CWin32"), WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, HWND_DESKTOP, NULL, hInstance, NULL);
  if (!hWnd)
  {
    return 1;
  }

  ShowWindow(hWnd, nCmdShow);
  UpdateWindow(hWnd);

  while ((ret = GetMessage(&msg, NULL, 0, 0)) != 0)
  {
    if (ret == -1)
    {
      return 1;
    }
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }

  return msg.wParam;
}

LRESULT CALLBACK WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
  switch (uMsg)
  {
    HANDLE_MSG(hWnd, WM_CREATE, MainWindow_OnCreate);
    HANDLE_MSG(hWnd, WM_DESTROY, MainWindow_OnDestroy);
    HANDLE_MSG(hWnd, WM_SIZE, MainWindow_OnSize);
    HANDLE_MSG(hWnd, WM_PAINT, MainWindow_OnPaint);
  default:
    return DefWindowProc(hWnd, uMsg, wParam, lParam);
  }
}

BOOL MainWindow_OnCreate(HWND hWnd, LPCREATESTRUCT lpCreateStruct)
{
  BufferedPaintInit();
  return TRUE;
}

void MainWindow_OnDestroy(HWND hWnd)
{
  BufferedPaintUnInit();
  PostQuitMessage(0);
}

void MainWindow_OnSize(HWND hWnd, UINT state, int cx, int cy)
{
  InvalidateRect(hWnd, NULL, FALSE);
}

void MainWindow_OnPaint(HWND hWnd)
{
  PAINTSTRUCT ps;
  HPAINTBUFFER hpb;
  HDC hdc;

  BeginPaint(hWnd, &ps);
  hpb = BeginBufferedPaint(ps.hdc, &ps.rcPaint, BPBF_COMPATIBLEBITMAP, NULL, &hdc);

  FillRect(hdc, &ps.rcPaint, GetStockBrush(DKGRAY_BRUSH));
  Sleep(320); // This simulates some slow drawing actions.

  EndBufferedPaint(hpb, TRUE);
  EndPaint(hWnd, &ps);
}

Is it possible to eliminate the flickering?

Answer 1

When the window is updated during a drag operation, then the OS has to show something in the extended window region. If you can’t provide anything then it will show the background until you do. Since you didn’t specify any background you get blackness. Surely you ought to be specifying a background brush? Simply adding the following to your code makes the behaviour more palatable:

wcex.hbrBackground = GetStockBrush(DKGRAY_BRUSH);

However, if you take as long as 320ms to respond to a WM_PAINT then you ruin the resize UI for the user. It becomes jerky and unresponsive. The system is designed around the assumption that you can paint the window quickly enough for dragging to feel smooth. The right way to fix your problem is to make WM_PAINT run in a reasonable time.

If you really can’t achieve quick enough painting for smooth dragging then I suggest a couple of alternatives:

1. Disable window updates during dragging. I’m sure this can be done for individual windows, but I can’t remember how to do it off the top of my head.
2. Paint something fake whilst a resize/drag is active, and postpone the real painting until when the resize/drag has completed. Listening for WM_ENTERSIZEMOVE and WM_EXITSIZEMOVE are the keys to this. This Microsoft sample program illustrates how to do that: https://github.com/microsoft/Windows-classic-samples/blob/master/Samples/Win7Samples/winui/fulldrag/