Is there any alternative cross-platform code on this one since System.Drawing.Common only supported on Windows?

[GeraldEugenio]

SharpAvi/Sample/Recorder.cs

Line 222 in 02b1447

private void GetScreenshot(byte[] buffer)

[baSSiLL]

Getting a screenshot is an OS-specific operation, so any cross-platform solution would do it via the native interop on each platform.
Check cross-platform UI frameworks for .NET (like Avalonia, MAUI), some of them probably contain this function.
If you're actually interested in a snapshot of a window of your application, then most UI frameworks should support this.