The window you are describing is called the "Taskbar preview", and it's a feature in Windows 11 that shows open applications or windows when you hover over their icon in the taskbar. To disable this feature, please follow these steps:
1. Right-click on the taskbar.
2. Select "Taskbar settings".
3. Scroll down and find the "Taskbar behaviors" section.
4. Toggle off the switch next to "Hover over app icon to see window previews".
This should turn off the Taskbar preview feature and stop the window from popping up spontaneously on the left side of your screen.