Both MoveWindow and SetWindowPos result in incorrect window position/size

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06-10-2024

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Why are MoveWindow and SetWindowPos Not Working as Expected?

Have you ever encountered a situation where your Windows application window refuses to move or resize correctly, even when using MoveWindow or SetWindowPos functions? This frustrating issue can arise from various underlying causes, often leading to an incorrect window position or size. This article delves into the common pitfalls associated with these functions, providing insights and solutions to help you overcome these challenges.

Scenario:

Let's imagine you're building a simple application that involves positioning and resizing a window programmatically. You might use the following code:

#include <windows.h>

int main() {
  // Create a window
  HWND hWnd = CreateWindow(L"BUTTON", L"My Window", WS_OVERLAPPEDWINDOW,
                           100, 100, 200, 100, NULL, NULL, NULL, NULL);

  // Attempt to move and resize the window
  MoveWindow(hWnd, 200, 200, 300, 200, TRUE);

  // Show the window
  ShowWindow(hWnd, SW_SHOW);

  // Message loop
  MSG msg;
  while (GetMessage(&msg, NULL, 0, 0)) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
  }

  return 0;
}

This code attempts to create a window at position (100, 100) with dimensions 200x100 and then move it to position (200, 200) with size 300x200. However, you might observe that the window does not end up in the expected location or with the desired dimensions.

Possible Causes and Solutions:

1. Window Styles: The window style you choose can heavily influence its positioning and resizing behavior. For instance, WS_OVERLAPPEDWINDOW includes the title bar, borders, and menu, contributing to the overall size of the window. If you are not accounting for these elements in your calculations, the window may end up smaller or larger than intended.

   Solution: Ensure you are using appropriate window styles that reflect your desired window layout. Consider using WS_POPUP or WS_CHILD if you require precise control over the window's boundaries.

2. Window Client Area vs. Window Rectangle: MoveWindow and SetWindowPos work with the window rectangle, which includes the title bar, borders, and menu. The client area, where your application content resides, is smaller. If you are calculating positions and sizes based on the client area, you need to account for the difference.

   Solution: Use GetClientRect and GetWindowRect to obtain the correct sizes of the client area and window rectangle, respectively, for accurate calculations.

3. Multiple Monitors: If your system has multiple monitors, the coordinate system might not behave as expected. The origin of the coordinate system is at the top-left corner of the primary monitor, and coordinates are relative to this origin.

   Solution: If your application needs to position windows across different monitors, consider using MonitorFromWindow and GetMonitorInfo to obtain the correct coordinates and adjust your calculations accordingly.

4. Window Minimization: When a window is minimized, its position and size are not considered. If you attempt to manipulate these attributes while the window is minimized, the changes might not be reflected when the window is restored.

   Solution: Ensure the window is in a normal state (not minimized) before using MoveWindow or SetWindowPos.

5. Window Decorations: Elements like scrollbars, status bars, and toolbars can affect the overall window size. These decorations might not be immediately accounted for by MoveWindow or SetWindowPos.

   Solution: Use the AdjustWindowRectEx function to calculate the size of the window rectangle taking into account the various decorations.

6. Window Parent/Child Relationships: If a window is a child of another window, its position and size are relative to its parent. Be aware of these relationships and adjust your calculations accordingly.

   Solution: Ensure you are using the correct parent window handle when positioning and resizing child windows.

Additional Tips:

• Debug: Use the Debug function (OutputDebugString) to print the window's current coordinates and dimensions to verify the actual values.
• Visualize: Use a tool like Spy++ (included with Visual Studio) to examine the window's attributes and the hierarchy of windows.
• Consider Alternatives: Depending on your specific needs, you might explore alternative solutions like using SetWindowPlacement or SetWindowLong to achieve precise window control.

By understanding these potential pitfalls and applying the recommended solutions, you can overcome challenges related to MoveWindow and SetWindowPos, resulting in a more predictable and reliable window positioning and resizing behavior in your Windows applications.