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By default, using Linux Mint with Cinnamon, clicking onto the title bar of a window with the middle mouse button puts it into the background. This can be especially annoying when, for example, quickly closing tabs in the browser (Google Chrome, Mozilla Firefox, etc.) and then accidentally middle-clicking the tab bar. Luckily, the fix is easy.
Head to the System Settings. From there, navigate to "Windows".
Now, under the "Titlebar" tab in the "Actions" section, you can configure the behaviour of middle-clicking the title bar with the "Action on title bar middle-click" option. "Lower" is the default setting, which causes the issue. I prefer to set it to "None".