right-window | Utility to move the focus between windows

I'm using a game engine library (AppGameKit) and Qt (5.12.3) in Visual Studio 2017. Normally using the default project template of this engine will build an executable that launches it's own window and handles key and mouse input.

Now I'm trying to tweak this to be used in Qt (embedded in a QWidget). For now I've managed to do so by passing the HWND of the QWidget (QWidget::winId()) to the game engine initialization function.

My only problem now, is that Qt is handling all input (mouse and keyboard), and I need the game engine to get access to it.

By inspecting the game engine's core code, I see the following WndProc function:

I can't detect precisely when the left mouse button is pressed with C++ in Windows 10 .

I tried 2 ways :

1. catching WM_LBUTTONDOWN message

2. using directly GetKeyState(VK_LBUTTON)

Each time, the behavior is the same :
If I press the left button during one second,

1. the WM_LBUTTONDOWN is sent about 0.5 seconds after I pressed down the left button

2. the GetKeyState(VK_LBUTTON) value returned changes exactly when WM_LBUTTONDOWN is sent, that means 0.5 seconds after I pressed the mouse button

the WM_LBUTTONDOWN is sent :

1. if I only click with the left button
2. if I press mouse down while moving the mouse

But I still don't know how to detect immediately when the left button is down in the case where I press the left button down a long time without moving the mouse. Is there an event I don't know about? Does Windows force us to think its way and use its "CLICK" and its "MOUSEDOWN" ? I mean, is there no way with windows to detect exactly when the mouse button is pressed ? How to detect precisely when the left button is pressed with C++ in windows ?

EDIT :

Thank you for your answers. I made a Minimal Complete and Verifiable code to show you.

The program below compiles with Visual C++ 2017 It shows a blue rectangle that moves each time GetMessage gets a message. The rectangle becomes red and is translated down when WM_LBUTTONDOWN is detected.

As I described in my first post : you will see that the rectangle becomes red when you click or when you press the mouse button down while moving the mouse, but it takes about 0.5 seconds to become red if you press down the left button and don't move the mouse.

Thank you again for your help.

Here is the code :

Below is my CSS and HTML code. As you can see the margin on right is not coming.

1. Can anybody tell me the reason for this?
2. Is the structure of HTML, CSS right? I have to show two windows in the middle of the page and a footer and a header. So, I have positioned everything absolute.

Is that correct practice?

I've started Direct2D from a very simple example. Acquire the factory and ID2D1HwndRenderTarget, then handle WM_PAINT message to draw just a background with a solid color using "Clear" function.

It's work fine, until I start to move the window. When the window is moving it turns gray like nothing is drawing. I've tried to draw an ellipse, and the result is the same.

How could one present the window content with the window moving?

P.S. In case the code is needed