Merge All Project Windows is not visible in the Window menu

Problem

There is no menu item "Merge All Project Windows" in the "Windows menu" but it should be there.

Resolution

• in Rider press `Shift+Shift`
• type `Registry`
• in the opened window, find the `ide.mac.bigsur.window.with.tabs.enabled` flag (you can start typing it in this window to find it quicker)
• enable this flag