I suspect some software "glitch".
First: Source for CSGO?
Second: does he have any additional software installed that enhances or purports to enhance game play performance?
Third: does the problem ever occur with just a single monitor? And occurs only with one specific monitor or the other - not both?
Use Task Manager and Resource Monitor to observe what all is running on his computer. Some unexpected and/or unidentified process running in the background? Do not start just disabling or uninstalling any such processes. Try to identify what they are and may be doing before taking any action.
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The desktop: Windows, Snap Assist and virtual desktops
These shortcuts control how individual windows act on your desktop, including virtual desktops.
- Windows key + M: Minimize all open windows.
- Windows key + Shift + M: Restore minimized windows.
- Windows key + Home: Minimize all windows except the selected or currently active window.
- Windows key + Up arrow: Maximize selected window.
- Windows key + Shift + Up arrow: Maximizes the active window vertically while maintaining its width.
- Windows key + Down arrow: Minimize selected window.
- Windows key + Left arrow or Right arrow: Snap selected window to the left or right half of the screen. (If the window is already there, Windows key + Up or Down will snap it into a quadrant.)
- Windows key + Shift + Left arrow or Right arrow: Move selected window to the left or right monitor.
- Windows key + Tab: Open Task view (virtual desktops).
- Windows key + Ctrl + D: Add new virtual desktop.
- Windows key + Ctrl + Right arrow: Move to the next virtual desktop (to the right).
- Windows key + Ctrl + Left arrow: Move to the previous virtual desktop (to the left).
- Windows key + Ctrl + F4: Close current virtual desktop.
Source: https://www.cnet.com/tech/computing/60-windows-10-keyboard-shortcuts-to-save-you-time/
When the screen minimizes determine if Windows key + Shift + M (second bullet in the above list) restores the screen.
Determine likewise if the other Windows key screen sizing combinations work and function as described.
Perhaps somewhere the mouse "left click" has been replaced with a "
Windows key + Down arrow: Minimize selected window" macro.
Look for any related keyboard or mouse related software that includes being able to do customized mouse and keyboard reconfiguration.