When you launch a Windows desktop application, the system changes the arrow to one with an animated busy spinner (presumably from IDI_ARROW to IDI_APPSTARTING). Once the application presents a window, the cursor is restored to indicate that it's ready for input.
With my own Win32 applications, the spinner continues for several (5?) seconds, even though the main window is fully rendered and ready for interaction. It was my understanding that the spinner vanishes once the just-launched program started pumping messages. More specifically, I thought it used the same criteria as WaitForInputIdle.
Since Windows 10, however, some applications, including ones I've written, appear to be stuck with the busy cursor for the duration of the timeout—even beyond the moment that the application becomes responsive to mouse and keyboard input.
What should I do in my programs to let the system know that initialization is complete and the spinner is no longer needed?
UPDATE: The problem occurs only when launching the application from the keyboard (like a CMD prompt, the Run window, or using the keyboard to launch the process in the debugger in Visual Studio). If you double-click the program with the mouse, the spinner vanishes quickly, as expected.
UPDATE 2: Given that others cannot repro, my best guess is that there's a buggy driver involved. Thanks to everyone who helped brainstorm in the comments.
Self-contained repro below.
#include <Windows.h>
LRESULT OnPaint(HWND hwnd, UINT, WPARAM, LPARAM) {
PAINTSTRUCT ps;
::BeginPaint(hwnd, &ps);
RECT rc;
::GetClientRect(hwnd, &rc);
::DrawTextW(ps.hdc, L"Hello, World", -1, &rc,
DT_SINGLELINE | DT_CENTER | DT_VCENTER | DT_NOPREFIX);
::EndPaint(hwnd, &ps);
return 0;
}
LRESULT MyWindowProc(HWND hwnd, UINT msg, WPARAM wp, LPARAM lp) {
switch (msg) {
case WM_DESTROY: ::PostQuitMessage(0); break;
case WM_PAINT: return OnPaint(hwnd, msg, wp, lp);
}
return ::DefWindowProcW(hwnd, msg, wp, lp);
}
int WinMain(HINSTANCE hinst, HINSTANCE, LPSTR, int nShowCmd) {
WNDCLASSW wc = {0};
wc.style = CS_HREDRAW | CS_VREDRAW;
wc.lpfnWndProc = MyWindowProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hinst;
wc.hIcon = NULL;
wc.hCursor = ::LoadCursorW(NULL, IDC_ARROW);
wc.hbrBackground = ::GetSysColorBrush(COLOR_WINDOW);
wc.lpszMenuName = nullptr;
wc.lpszClassName = L"My Very Own Window Class";
const ATOM atomClass = ::RegisterClassW(&wc);
if (atomClass == 0) return ::GetLastError();
// Ugly cast for WinAPI's legacy type punning.
const LPCWSTR wndclass =
reinterpret_cast<LPCWSTR>(static_cast<UINT_PTR>(atomClass));
const HWND hwnd =
::CreateWindowW(wndclass, L"Test", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT,
CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL,
NULL, hinst, nullptr);
if (hwnd == NULL) return ::GetLastError();
::ShowWindow(hwnd, nShowCmd);
MSG msg = {0};
while (::GetMessageW(&msg, NULL, 0, 0) > 0) {
::TranslateMessage(&msg);
::DispatchMessageW(&msg);
}
return (msg.message == WM_QUIT) ? static_cast<int>(msg.wParam) : 1;
}
What should I do in my programs to let the system know that initialization is complete and the spinner is no longer needed?
There seems to be agreement that the spinner should vanish once the application has started pumping messages and has no pending input events queue.
The fact that's not working in all cases for me is likely a local issue, like a buggy driver spamming the input queue (which somehow never make it to window messages). There's not much point in hashing that out here on SO, so I'm putting up this community wiki answer and moving on.
I was looking into why when running my Windows App, it has a short flash of white background before rendering the actual app (i.e., before WM_ERASEBKGND and WM_PAINT is received).
Now, I just noticed that this problem is also present in the default sample app created by Visual Studio. At least this is the case for me when running under Windows 10,21H1 (in VS2008 & VS2013).
The only thing you have to do, after creating a "new Win32 Project", is change the background color of the window class, e.g., to the color red:
//wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wcex.hbrBackground = (HBRUSH) CreateSolidBrush(RGB(255, 0, 0));
And then add a WM_ERASEBKGND with a Sleep to the WndProc:
case WM_PAINT:
hdc = BeginPaint(hWnd, &ps);
// TODO: Add any drawing code here...
EndPaint(hWnd, &ps);
break;
case WM_ERASEBKGND:
Sleep(1000);
return DefWindowProc(hWnd, message, wParam, lParam);
The Sleep exaggerates the problem, causing the white background to show for at least a second.
After that the red background is drawn as expected.
I'm including a short video when running the App with those changes.
For any app, it looks quite unprofessional that the window flashes white before rendering,
especially if the interface is dark.
So my question is: What is causing this behavior?
The background color is set through RegisterClassEx and passed to CreateWindow, before calling ShowWindow(..) So Windows should know that the background color is red. So why does it render it white? Am I missing something?
Ideally, I would like to change this initial background color to something other than white, such as black. But how? I've tried drawing to the window before calling ShowWindow, without luck.
This indeed seems to be a Windows bug as demonstrated by the excellent research by the OP.
The bug is even affecting applications developed by Microsoft.
The question is what is best workaround, especially for products that need to support backwards compatibility even after a fix is released in a specific version of Windows 11 (or Windows 10).
The main problem is that it is the act of making the window visible that makes Windows paint it with the white brush prior to correctly applying the background brush, regardless of what was painted into its DC beforehand. Therefore tricks such as painting into the DC prior to showing the window are unsatisfying, as the white background will still be shown, even if only for a few frames.
One method that seems to work well is to make the window visible, but fully transparent, paint the background, and then make the window opaque. We also need to animate the activation of the window, so it doesn't just pop in. For example, we can hijack WM_SHOWWINDOW for this:
case WM_SHOWWINDOW:
{
if (!GetLayeredWindowAttributes(hWnd, NULL, NULL, NULL))
{
SetLayeredWindowAttributes(hWnd, 0, 0, LWA_ALPHA);
DefWindowProc(hWnd, WM_ERASEBKGND, (WPARAM)GetDC(hWnd), lParam);
SetLayeredWindowAttributes(hWnd, 0, 255, LWA_ALPHA);
AnimateWindow(hWnd, 200, AW_ACTIVATE|AW_BLEND);
return 0;
}
return DefWindowProc(hWnd, message, wParam, lParam);
}
break;
Full sample code:
#include "framework.h"
#include "WindowsProject1.h"
#define MAX_LOADSTRING 100
HINSTANCE hInst;
WCHAR szTitle[MAX_LOADSTRING];
WCHAR szWindowClass[MAX_LOADSTRING];
ATOM MyRegisterClass(HINSTANCE hInstance);
BOOL InitInstance(HINSTANCE, int);
LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK About(HWND, UINT, WPARAM, LPARAM);
HINSTANCE mInstance;
int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
_In_opt_ HINSTANCE hPrevInstance,
_In_ LPWSTR lpCmdLine,
_In_ int nCmdShow)
{
UNREFERENCED_PARAMETER(hPrevInstance);
UNREFERENCED_PARAMETER(lpCmdLine);
mInstance = hInstance;
LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
LoadStringW(hInstance, IDC_WINDOWSPROJECT1, szWindowClass, MAX_LOADSTRING);
MyRegisterClass(hInstance);
if (!InitInstance (hInstance, nCmdShow))
{
return FALSE;
}
HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WINDOWSPROJECT1));
MSG msg;
while (GetMessage(&msg, nullptr, 0, 0))
{
if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
}
return (int) msg.wParam;
}
ATOM MyRegisterClass(HINSTANCE hInstance)
{
WNDCLASSEXW wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW | CS_CLASSDC;
wcex.lpfnWndProc = WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WINDOWSPROJECT1));
wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);
wcex.hbrBackground = CreateSolidBrush(RGB(255, 0, 0));
wcex.lpszMenuName = MAKEINTRESOURCEW(IDC_WINDOWSPROJECT1);
wcex.lpszClassName = szWindowClass;
wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
return RegisterClassExW(&wcex);
}
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
hInst = hInstance;
HWND hWnd = CreateWindowExW(WS_EX_LAYERED, szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);
if (!hWnd)
{
return FALSE;
}
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
return TRUE;
}
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
switch (message)
{
case WM_COMMAND:
{
int wmId = LOWORD(wParam);
switch (wmId)
{
case IDM_ABOUT:
DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
break;
case IDM_EXIT:
DestroyWindow(hWnd);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
}
break;
case WM_SHOWWINDOW:
{
if (!GetLayeredWindowAttributes(hWnd, NULL, NULL, NULL))
{
SetLayeredWindowAttributes(hWnd, 0, 0, LWA_ALPHA);
DefWindowProc(hWnd, WM_ERASEBKGND, (WPARAM)GetDC(hWnd), lParam);
SetLayeredWindowAttributes(hWnd, 0, 255, LWA_ALPHA);
AnimateWindow(hWnd, 200, AW_ACTIVATE|AW_BLEND);
return 0;
}
return DefWindowProc(hWnd, message, wParam, lParam);
}
break;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
ReleaseDC(hWnd, hdc);
EndPaint(hWnd, &ps);
}
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, message, wParam, lParam);
}
return 0;
}
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
UNREFERENCED_PARAMETER(lParam);
switch (message)
{
case WM_INITDIALOG:
return (INT_PTR)TRUE;
case WM_COMMAND:
if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
{
EndDialog(hDlg, LOWORD(wParam));
return (INT_PTR)TRUE;
}
break;
}
return (INT_PTR)FALSE;
}
A more controversial answer could be that this is simply a bug in Windows.
For reference, (aside from the existing GIFs from Windows 10 that I already posted) here are recordings of the sample app running with and without background erase in Windows XP, Windows 7 and Windows 11.
Windows XP:
Windows XP: Without WM_ERASEBKGND/WM_PAINT: OK (no white background)
Windows XP: With WM_ERASEBKGND: OK (no white background)
Windows 7:
Windows 7: Without WM_ERASEBKGND/WM_PAINT: NOT OK (white background)
Windows 7: With WM_ERASEBKGND: NOT OK (white background)
Windows 7: With WM_ERASEBKGND + Sleep: NOT OK (white background)
Windows 7 with Aero disabled:
Windows 7 with Aero disabled: Without WM_ERASEBKGND/WM_PAINT: OK (no white background)
Windows 7 with Aero disabled: With WM_ERASEBKGND: OK (no white background)
Windows 7 with Aero disabled: With WM_ERASEBKGND + Sleep: OK (no white background)
Windows 11 (with Animation disabled):
Windows 11: Without WM_ERASEBKGND/WM_PAINT: NOT OK (white background)
Windows 11: With WM_ERASEBKGND: OK (no white background)
Windows 11: With WM_ERASEBKGND + Sleep: NOT OK (white background)
I've added Sleep to tests where it was hard to see the issue.
To sum up:
Windows XP: No issue. Everything seems to work as expected.
Windows 7: Issue occurs when Aero is enabled (Windows 7
theme), but not when it is disabled (Classic theme).
Windows 10: Issue occurs for all tests.
Windows 11: Issue occurs, but works without Sleep added.
Most likely since this was running on a faster machine.
So although I cannot conclude anything solid from these tests, it does look like this behavior was introduced in Windows 7 with Aero.
If someone can debunk this claim, please comment below.
I've hit this issue recently. I tried mnistic's solution using layered windows and transparency but it caused problems with rendering pane captions in the MFC app I'm working on. However, I've found a simple solution which appears to work nicely without any need for animation, changing window styles, etc.:
The Desktop Window Manager API enables a window to be "cloaked", so that it isn't shown on the screen but is still internally composited, i.e. still accumulates the results of drawing operations. You can turn "cloaking" on by:
BOOL cloak = TRUE;
DwmSetWindowAttribute(hwnd, DWMWA_CLOAK, &cloak, sizeof(cloak));
To avoid the white flash on first showing the window, do the above before calling ShowWindow(). Then do the initial UpdateWindow() to get correct content drawn. Finally, turn "cloaking" off using:
BOOL cloak = FALSE;
DwmSetWindowAttribute(hwnd, DWMWA_CLOAK, &cloak, sizeof(cloak));
to get the final window content displayed.
This should work on all Windows versions which have a Desktop Windows Manager, so Windows Vista and up.
Did some more poking, so here is a different potential answer.
I realized that even if I completely discard WM_PAINT and WM_ERASEBKGND (i.e., return 0 in WM_PAINT and return TRUE in WM_ERASEBKGND), I can still get the app to draw the red background by manually resizing the window!
Here is a clip to illustrate:
This means that Windows does indeed know and respect hbrBackground, which is great! For some odd reason it just doesn't clear it to that, but to white instead.
(Incidentally, I went through all the system colors in the registry (HKEY_CURRENT_USER\Control Panel\Colors HKEY_CURRENT_USER\Control Panel\Desktop\Colors) with a "255 255 255" setting and forcibly changed them to see if that would change the initial white background. But no luck. This makes me conclude that the white background is not a system color.)
Anyway, the above lead me to try to programmatically resize the window after ShowWindow. But since I don't want it to flicker on open, do the ShowWindow off-screen.
So here is the code that would replace regular ShowWindow(..):
int x0 = GetSystemMetrics(SM_XVIRTUALSCREEN);
int x1 = GetSystemMetrics(SM_CXVIRTUALSCREEN);
RECT rect;
GetWindowRect(hWnd, &rect);
// resize and move off-screen
SetWindowPos(hWnd, NULL, x1-x0, 0, 0, 0, SWP_NOREDRAW );
// show window
ShowWindow(hWnd,nCmdShow);
// restore and redraw
SetWindowPos(hWnd, NULL, rect.left, rect.top, rect.right-rect.left, rect.bottom-rect.top, 0 );
Now, I would call this a hack. Yet, it does not rely on WM_ERASEBKGND nor WM_PAINT, so there should be less of a timing issue.
Also, the window shows up exactly like a regular ShowWindow(...) would, just with the correct hbrBackground, which is what I wanted.
Here is what it looks like # 25Hz:
Notice that there is no flash of white background.
Please note that I've tried to write the code to cater for virtual desktop/multi-monitor, but haven't actually tested that.
But unfortunately everything is not fine and dandy. As I was writing this answer, I did quite a few trial runs with OBSStudio recording # 60Hz, and went through the footage. There I found one that simply shows trash inside the window frame on open (apparently from Chrome), for just one frame. Here is a slowed-down replay:
I'm stumped. Perhaps that is the real issue ?
I did some more testing, and want to post a potential answer to this question.
Now, this is mainly based on the suggestion by #JonathanPotter, so full credit to him. And while it doesn't really fix the problem, it does alleviate it quite a bit.
Now, ideally, it would be great if Windows would simply render the window with the correct initial background color, but no matter how hard I've tried, I can only get it to update the background color by utilizing WM_ERASEBKGND or WM_PAINT.
So it seems that the time delay between showing the window (i.e. using ShowWindow), and the actual clearing of the background (WM_ERASEBKGND) is the crux of the problem. Hence, it makes sense to profile it. I've done so by recording the time difference between calling ShowWindow and reaching WM_ERASEBKGND using QueryPerformanceCounter.
So on an i7-4960HQ CPU # 2.60GHz running Window 10, the time between ShowWindow and WM_ERASEBKGND is between 100 - 317ms. It fluctuates quite a bit. This is with a vanilla Win32 Sample App, built in Release without any Sleeps or anything like that, but using a red hbrBackground to show the issue. This means that the white background is clearly visible for a few frames before the red background is drawn. Here is an animated gif captured # 25Hz: The white background is visible for 3 frames in that animation.
Now the potential fix is to use a combination of SetWindowPos and RedrawWindow before showing the window.
For my tests I simply added these two lines before calling ShowWindow(..):
SetWindowPos(hWnd, NULL, 0,0,0,0, SWP_NOMOVE | SWP_NOSIZE | SWP_NOREDRAW);
RedrawWindow(hWnd, NULL, 0, RDW_INVALIDATE | RDW_ERASE);
Although RedrawWindow does not seem to make any difference.
Profiling again, the time between ShowWindow and WM_ERASEBKGND is now 10 - 23ms. A 10x speed-up!
Again, an animated gif captured (with SetWindowPos) # 25Hz:
This clearly shows that the flash of white background is gone, and thus the problem is fixed. It's like night and day.
Now, I would argue that this is not a fix, but rather a workaround. Since the underlying problem of Windows using a white background color is still there.
And since this is a timing issue, I can easily imagine that the white background could show up again, say if the system was sluggish or busy doing other stuff.
Similarly, having a faster system means you that you are less likely to see this in the first place, effectively hiding the issue.
But simply setting a breakpoint in WM_ERASEBKGND will still show you a white window.
Also, I have no explanation for the speed-up. I tracked the number of messages in the message pump, and they are the same in both scenarios.
Now, I'm still hoping for a better fix. I find it hard to believe that the Microsoft engineers found it cool to fill all freshly created Windows with a hardcoded 0xFFFFFF, so I'm hoping that this color is actually read from somewhere, and thus possible to change, so the initial background matches the hbrBackground.
Please feel free to post alternative answers, questions, or suggestions.
I will of course update this thread if I figure out anything else.
I'm saving/restoring my window position using GetWindowPlacement/SetWindowPlacement on Windows 10. My application is DPI aware. The issue occurs when SetWindowPlacement is both sizing and moving the window from monitor #1 with one DPI to monitor #2 with a different DPI. The coordinates have been saved as the correct size for monitor #2 in the WINDOWPLACEMENT structure.
The window is first resized during SetWindowPlacement while it's still on monitor #1. Then window is moved to monitor #2, which causes a WM_DPICHANGED message to fire, saying the window size should be changed. The suggested size is incorrect since it's changing the size of the window which was already the correct size for monitor #2.
What is the correct way to solve this? Should I be setting a flag before SetWindowPlacement to ignore WM_DPICHANGED messages until that call is done? Are there cases where that will cause me to miss a message I shouldn't be ignoring?
Thanks
Edit: Attached repro for #SongZhu-MSFT.
In this test case I'm using a Surface Studio 2 as my primary monitor, running at 4500x3000, using 175% scaling. On the right of that monitor, aligned to the bottom is a 1920x1080 monitor set to 100% scaling. This code attempts to open the monitor on the right monitor with a set size, however a DPICHANGE message comes through during the SetWindowPlacement() call which causes the size to be adjusted incorrectly, unless I manually avoid it. Sample code is edited from:
https://learn.microsoft.com/en-us/windows/win32/learnwin32/windows-hello-world-sample
#ifndef UNICODE
#define UNICODE
#endif
#include <windows.h>
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE, PWSTR pCmdLine, int nCmdShow)
{
// Register the window class.
const wchar_t CLASS_NAME[] = L"Sample Window Class";
WNDCLASS wc = { };
wc.lpfnWndProc = WindowProc;
wc.hInstance = hInstance;
wc.lpszClassName = CLASS_NAME;
RegisterClass(&wc);
// Create the window.
HWND hwnd = CreateWindowEx(
0, // Optional window styles.
CLASS_NAME, // Window class
L"Learn to Program Windows", // Window text
WS_OVERLAPPEDWINDOW, // Window style
// Size and position
CW_USEDEFAULT, CW_USEDEFAULT, 1280, 720,
NULL, // Parent window
NULL, // Menu
hInstance, // Instance handle
NULL // Additional application data
);
if (hwnd == NULL)
{
return 0;
}
WINDOWPLACEMENT wp = {};
wp.length = sizeof(wp);
wp.showCmd = 1;
wp.ptMaxPosition.x = -1;
wp.ptMaxPosition.y = -1;
wp.ptMinPosition.x = -1;
wp.ptMinPosition.y = -1;
wp.rcNormalPosition.left = 4510;
wp.rcNormalPosition.top = 2320;
wp.rcNormalPosition.right = wp.rcNormalPosition.left + 1850;
wp.rcNormalPosition.bottom = 2909;
::SetWindowPlacement((HWND)hwnd, &wp);
ShowWindow(hwnd, nCmdShow);
// Run the message loop.
MSG msg = { };
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
switch (uMsg)
{
case WM_DESTROY:
PostQuitMessage(0);
return 0;
case WM_DPICHANGED:
{
int dpi = HIWORD(wParam);
{
RECT* const prcNewWindow = (RECT*)lParam;
SetWindowPos(hwnd,
NULL,
prcNewWindow->left,
prcNewWindow->top,
prcNewWindow->right - prcNewWindow->left,
prcNewWindow->bottom - prcNewWindow->top,
SWP_NOZORDER | SWP_NOACTIVATE);
}
return 0;
}
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
// All painting occurs here, between BeginPaint and EndPaint.
FillRect(hdc, &ps.rcPaint, (HBRUSH) (COLOR_WINDOW+1));
EndPaint(hwnd, &ps);
}
return 0;
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
And the .manifest I'm using.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<application xmlns="urn:schemas-microsoft-com:asm.v3">
<windowsSettings>
<dpiAwareness xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">PerMonitorV2,permonitor</dpiAwareness>
<dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true/pm</dpiAware>
</windowsSettings>
</application>
</assembly>
I encountered the same issue in a slightly different context: saving and restoring the positions of other applications' windows (i.e. when un-docking a laptop all windows move to the primary display, but when re-docking we'd like them to go back where they were).
Since I don't control the third-party application windows, neither of #Sunius' suggestions work: I cannot make them ignore WM_DPICHANGED, nor can I know whether they will indeed react to WM_DPICHANGED -- if they aren't actually DPI-aware applications then pre-adjusting the window size is unnecessary and counterproductive.
My solution so far is a little clumsy, but simple and effective: when moving windows between different-DPI monitors, simply call SetWindowPlacement() twice. The first call will put it on the right monitor, but possibly with the wrong size, but the second call will immediately fix the size since it's already in the correct position.
The only catch here is that GetDpiForWindow() appeared to be unreliable for some applications, especially if the window remained minimized (I think Windows internally doesn't update the window's DPI setting for its new monitor if it's minimized). So instead I had to use MonitorFromWindow() and then GetDpiForMonitor() to detect when a window is going to change DPI, in order to trigger the second SetWindowPlacement().
There are two ways to solve this:
Before calling SetWindowPlacement, set a flag like s_IsInsideWindowMove to true, and if it's set when WM_DPICHANGED fires, do not follow the suggestion to resize the window. Once SetWindowPlacement returns, set the flag back to false;
Calculate the size passed to SetWindowPlacement as if you were placing it on the monitor with the same DPI as the monitor the window is currently on. For instance, if you're moving the window from DPI 144 to DPI 192 monitor and you want the end result size to be 800x600, ask SetWindowPlacement for a size of 600x450.
We use the first option as it's just easier to reason about and implement.
I have encountered an annoying problem. When the mouse pointer is positioned over my main window and the owning popup window is shown (see example below) or is made invisible a WM_MOUSEMOVE message is generated each time even if the mouse has not been moved. For several reasons it can't be tolerated in my case.
hWnd = CreateWindowEx(0, wcx.lpszClassName, L"Demo", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, nullptr, 0, hInstance, nullptr);
HWND hWndPopupTest = CreateWindowEx(WS_EX_NOACTIVATE | WS_EX_TOPMOST, L"Static", L"DemoPopup", WS_POPUP | WS_VISIBLE, 10, 10, 100, 100, hWnd, 0, hInstance, nullptr);
ShowWindow(hWnd, SW_SHOW);
ShowWindow(hWndPopup, SW_SHOWNOACTIVATE);
Sleep(1000);
ShowWindow(hWndPopup, SW_HIDE);
The same behavior occurs when ReleaseCapture is called. Is this a feature that can be disabled? Is it a known "problem" or is there a workaround?
Edit: Dirty Workaround
In (main) window procedure we could test if the mouse position has changed since last WM_MOUSEMOVE. If the position has not changed it must be because wither a popup window was shown/hidden or a some window capture was released.
Based on the information provided here (thank you #IInspectable) my general solution would be to detect if the given point is a real point by looking in mouse position history with GetMouseMovePointsEx. If no point is found it means no valid movement occurred.
POINT CurrentPoint{GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam)};
MapWindowPoints(hWnd, nullptr, &CurrentPoint, 1);
MOUSEMOVEPOINT mmpi = {
CurrentPoint.x, CurrentPoint.y, GetTickCount(), 0
};
MOUSEMOVEPOINT mmpo = {0};
if (GetMouseMovePointsEx(sizeof(mmpi), &mmpi, &mmpo, 1, GMMP_USE_DISPLAY_POINTS) > 0) {
MyInstance->HandleMouseMove(POINT{GET_X_LPARAM(lParam), GET_Y_LPARAM(lParam)}, wParam);
} else {
// No mouse point found in history, so couldn't be a valid point
}
I know I am probably missing something, but I can't seem to get windows to show the live thumbnail preview correctly when using a window that has a region. When hitting the minimize button the preview will clip to the minimized size (160x25) rather than showing the full preview (like it does with other windows).
Few points to make:
1) The preview works fine in Windows Live Messenger, so Microsoft figured out a way to do it.
2) If I call SetWindowRgn only before a window is visible, it works fine (so its not a fault of the DWM not knowing how to deal with regioned windows.) I can call SetWindowRgn many times before the window is visible and it works great.
3) I need to set the window region after I show the window in case of a resize. So a fix to just set it before will not work.
4) Even when using the default window procedure, the bug still happens. So it is not a fault of processing a message incorrectly (but could be a fault of 'not processing' one :) )
5) When minimizing by clicking the taskbar button (instead of the minimize button in the window), the preview normally works fine (even after setting the region when visible). Again proving that it does not how to deal with the preview.
The bug happens if I set a region after I have shown the window. Code to follow:
void create(HINSTANCE hInst)
{
char* className = "default";
/* Register
*/
WNDCLASSEX wcex;
memset(&wcex,0,sizeof(wcex));
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = DefWindowProc;
wcex.hInstance = hInst;
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wcex.lpszClassName = className;
RegisterClassEx(&wcex);
/* Create
*/
HWND hwnd = CreateWindow(className, className, WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInst, NULL);
/*
* Set the region
* If set before the window is shown for the first time, minimize preview on vista works
*/
RECT rect;
GetWindowRect(hwnd,&rect);
HRGN rgn = CreateRoundRectRgn(0,0,rect.right-rect.left,rect.bottom-rect.top,15,15);
SetWindowRgn(hwnd,rgn,TRUE);
/* Show the window
*/
ShowWindow(hwnd,SW_SHOW);
/*
* Set the region a second time.
* Doing this will break minimize preview on vista
*/
rgn = CreateRoundRectRgn(0,0,rect.right-rect.left,rect.bottom-rect.top,35,35);
SetWindowRgn(hwnd,rgn,TRUE);
}
int APIENTRY _tWinMain(HINSTANCE hInstance,
HINSTANCE hPrevInstance,
LPTSTR lpCmdLine,
int nCmdShow)
{
MSG msg;
create(hInstance);
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int) msg.wParam;
}
Microsoft responded to a tech support incident and listed this as a bug within Vista.