Steps to Reproduce

class Form1 : Form {
    static void Main() {
        Application.Run(new Form1());
    }
    Form1() {
        var form = new Form {
            TopMost = true,
            ShowInTaskbar = false
        };
        form.Show();
        form.Hide();
        form.Show();
    }
}

Current Behavior

TopMost and ShowInTaskbar never work again.

On which platforms did you notice this

Linux
Linux 4.4.0-142-lowlatency #168-Ubuntu SMP PREEMPT Wed Jan 16 21:51:18 UTC 2019 x86_64

Version Used:

Mono JIT compiler version 5.18.0.240 (tarball Wed Jan 16 09:13:03 UTC 2019)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
TLS: __thread
SIGSEGV: altstack
Notifications: epoll
Architecture: amd64
Disabled: none
Misc: softdebug
Interpreter: yes
LLVM: yes(600)
Suspend: preemptive
GC: sgen (concurrent by default)