Copying from Mono port of Windows Forms doesn't work in some programs on Linux

[ForNeVeR]

Steps to Reproduce

1. Create a following program, compile it and run with Mono:

   using System.Windows.Forms;
   
   namespace ConsoleApplication1
   {
       internal class Program
       {
           public static void Main(string[] args)
           {
               var form = new Form();
               var textBox = new TextBox();
               form.Controls.Add(textBox);
               Application.Run(form);
           }
       }
   }

2. Enter any text into a text box, select it and press Ctrl-C (or other default keyboard shortcut for Copy action).

Current Behavior

1. Try to paste the result:

   • into gedit: success
   • into gnome-terminal: failure
   • into any Swing-based Java application: failure

2. Try to call xclip -selection clipboard -o -t TARGETS > targets.txt: it will output 40 bytes of garbage instead of a list of newline-separated targets supported by Mono.

Expected Behavior

1. Paste into gnome-terminal and into any Swing-based Java application should work by default.
2. xclip -selection clipboard -o -t TARGETS should output a list of newline-separated targets, not garbage.

On which platforms did you notice this

• Linux (I was testing on a 64-bit Ubuntu 18.04)

Version Used:

Latest master, commit 90a91429ae75240b4d0e791a563a9d553be09d56, and latest stable, version 5.20.1.19.

Analysis

The information provided by Mono into X11 clipboard mechanism is wrong in two ways:

1. Mono claims to provide the information of type TARGETS instead of type ATOM, so XGetWindowProperty will fail if asked to get information of type ATOM when performing a paste action.
2. The corresponding Mono code seems to think that X11 Atom is 32-bit always, but it is not the case: Atom is 64-bit on a 64-bit system.

I will send a patch fixing the issues soon.