Mono applications that are minimized to try, does not show the real tray icon but a fully-black tray icon instead.
This bug was first reported in the old bugzilla tracker, In particular affects KeePass, and the KeePass developers asked multiple times to report this to the Mono developers (1, 2,3)

Steps to Reproduce

1. Download KeePass from here,
2. Run mono KeePass.exe
3. A new systray icon is created, but it is full black instead of the KeePass icon

Current Behavior

Expected Behavior

On which platforms did you notice this

[ ] macOS
[X ] Linux
[ ] Windows

Version Used:

 mono --version
Mono JIT compiler version 4.6.2 (Debian 4.6.2.7+dfsg-1ubuntu1)
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 
        LLVM:          supported, not enabled.
        GC:            sgen