WinAPI CreateWindow WS_POPUP resize flickering - winapi

Question

I'm creating a window with no borders (WS_POPUP in CreateWindow) and need to resize it when mouse is over and change back when mouse is out of the application. It works OK, but it flickers when I change the position using SetWindowPos. I need to decrease y by 30 pixels and increase height, so the bottom of the window remains in the same position.
Below is the code:
WNDCLASS windowClass;
ZeroMemory(&windowClass, sizeof(WNDCLASS));
windowClass.style = 0;// CS_HREDRAW | CS_VREDRAW;
windowClass.lpfnWndProc = (WNDPROC) WindowProc;
windowClass.hInstance = hInst;
windowClass.lpszClassName = PROCESS_WINDOW_CLASS;
windowClass.hIcon = NULL;
windowClass.hCursor = LoadCursor(NULL, IDC_ARROW);
windowClass.hbrBackground = (HBRUSH)(COLOR_3DFACE + 1);
windowHandle = CreateWindowEx(
0,
PROCESS_WINDOW_CLASS,
L"",
WS_POPUP | WS_CLIPSIBLINGS,
x, //x
y, //y
width, //width
height, //height
NULL,
NULL,
hInst,
NULL);
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
switch (uMsg) {
case WM_MOUSEMOVE: {
if (!mouseOver) {
TRACKMOUSEEVENT tme;
tme.cbSize = sizeof(TRACKMOUSEEVENT);
tme.dwFlags = TME_LEAVE;
tme.hwndTrack = hwnd;
if (TrackMouseEvent(&tme)) {
RECT rect;
GetWindowRect(hwnd, &rect);
int y = rect.top;
int height = rect.bottom - y;
y -= BUTTONS_AREA_HEIGHT;
height += BUTTONS_AREA_HEIGHT;
//tried with MoveWindow, DeferWindowPos - same thing
//MoveWindow(hwnd, rect.left, y, rect.right - rect.left, height, TRUE);
//HDWP winPos = BeginDeferWindowPos(1);
//DeferWindowPos(winPos, hwnd, NULL, rect.left, y, rect.right - rect.left, height, 0);
//EndDeferWindowPos(winPos);
SetWindowPos(hwnd, NULL, rect.left, y, rect.right - rect.left, height,
SWP_NOZORDER | SWP_NOACTIVATE | SWP_NOREDRAW | SWP_NOSENDCHANGING);
rect.top = y;
rect.bottom = height - y;
InvalidateRect(hwnd, &rect, FALSE);
//0);
RedrawWindow(hwnd, NULL, NULL, RDW_INTERNALPAINT | RDW_INVALIDATE | RDW_NOERASE);
mouseOver = TRUE;
}
}
}
break;
case WM_MOUSELEAVE: {
RECT rect;
GetWindowRect(hwnd, &rect);
int y = rect.top;
int height = rect.bottom - y;
y += BUTTONS_AREA_HEIGHT;
height -= BUTTONS_AREA_HEIGHT;
MoveWindow(hwnd, rect.left, y, rect.right - rect.left, height, TRUE);
//SetWindowPos(hwnd, NULL, rect.left, y, rect.right - rect.left, height,
// SWP_NOZORDER | SWP_NOMOVE | SWP_NOACTIVATE);
//InvalidateRect(hwnd, &rect, FALSE);
//SetWindowPos(hwnd, NULL, rect.left, y, rect.right - rect.left, height, 0);
mouseOver = FALSE;
}
case WM_CREATE: {
LONG lExStyle = GetWindowLongPtr(hwnd, GWL_EXSTYLE);
lExStyle &= ~(WS_CAPTION | WS_THICKFRAME | WS_MINIMIZE | WS_MAXIMIZE |
WS_SYSMENU | WS_EX_DLGMODALFRAME | WS_EX_CLIENTEDGE | WS_EX_STATICEDGE);
SetWindowLongPtr(hwnd, GWL_EXSTYLE, lExStyle | WS_EX_TOOLWINDOW | WS_EX_TOPMOST);
SetWindowPos(hwnd, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOMOVE | SWP_NOSIZE);
}
return 0;
case WM_DESTROY:
PostQuitMessage(EXIT_SUCCESS);
break;
default:
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
return 0;
}
I tried all combinations of SetWindowPos, MoveWindow etc but when mouse is over the window, it seems that window is first settings Y position, then it increases the height after a few milliseconds and this is visible.
Does anyone know a solution for this?

Answer 1

I fixed the problem by using the SetWindowRgn function.
This still doesn't answer the question why flickering happens, but it fixed the problem for me as I was able to achieve the same thing. Code is below:
HRGN region = CreateRectRgn(0, BUTTONS_AREA_HEIGHT, width, height);
SetWindowRgn(windowHandle, region, TRUE);
DeleteObject(region);