How to deal with invalidly painted ComboBox control in win32 winapi?

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In the Win32 WinAPI, a ComboBox control can sometimes become "invalidly painted" due to various reasons, resulting in visual glitches or incorrect rendering. This can occur if the control is not properly redrawn or updated when its state changes. To deal with this issue, you can take the following steps:

  1. Ensure Proper Redrawing:
    Make sure that you are handling the WM_PAINT message correctly for the ComboBox control. In response to this message, you should use the BeginPaint and EndPaint functions to perform the necessary painting. Avoid painting directly in other message handlers.

  2. Handle WM_ERASEBKGND:
    In the ComboBox's parent window procedure, handle the WM_ERASEBKGND message by returning TRUE to indicate that you have erased the background of the ComboBox yourself. This prevents flickering and improves the control's appearance.

  3. Handle WM_CTLCOLORLISTBOX:
    When the drop-down list of the ComboBox is displayed, it uses a list box control. To ensure that the list box is properly painted, handle the WM_CTLCOLORLISTBOX message in the parent window procedure and set the background and text colors for the list box using the SetTextColor and SetBkColor functions.

  4. Refresh the ComboBox:
    If the ComboBox is not refreshing or updating correctly, you can manually trigger a refresh by sending the WM_PAINT message using the InvalidateRect function. For example, after changing the ComboBox's state or items, call InvalidateRect(hWndComboBox, NULL, TRUE) to request a repaint.

  5. Avoid Overlapping Drawing:
    If you are drawing custom content inside the ComboBox, ensure that it does not overlap with the standard ComboBox drawing. Overlapping drawing can lead to visual glitches. Use custom drawing techniques like owner-drawing (ODT_COMBOBOX) if necessary.

  6. Check Control Styles:
    Ensure that the control styles and attributes are set correctly for the ComboBox. Check for any conflicting or incorrect styles that may affect its appearance.

  7. Double-Buffering:
    Consider implementing double-buffering techniques to reduce flickering and improve the ComboBox's rendering. You can create an off-screen device context (bitmap) and draw to it before copying the content to the ComboBox's client area in one operation.

  8. UpdateComboBoxWindow (Optional):
    In some cases, especially when using custom drop-down content, you may need to handle additional messages or send specific messages (like CB_SHOWDROPDOWN) to update the ComboBox correctly.

By following these steps, you can address common issues related to invalidly painted ComboBox controls in the Win32 WinAPI. Proper handling of painting messages, updating control states, and handling custom drawing will help ensure that the ComboBox behaves as expected and appears correctly on the screen.