Spawn a thread to fake do our XDND polling

We cannot integrate this into our event loop like we planned, because
Wine a) grabs the mouse pointer so we cannot do that, and b) blocks the
GUI thread. So instead we will spawn our own thread and do polling based
XDND. When Wine's tracker window gets destroyed, we know that the left
mouse button has been released.

src/wine-host/xdnd-proxy.h

@@ -27,6 +27,9 @@
 #pragma pop_macro("_WIN32")
 
 #include <windows.h>
+#include <boost/container/small_vector.hpp>
+
+#include "utils.h"
 
 /**
  * A simple, unmapped 1x1 proxy window we'll use for our Wine->X11 drag-and-drop
@@ -52,8 +55,11 @@ class ProxyWindow {
 
    private:
     std::shared_ptr<xcb_connection_t> x11_connection;
+
+   public:
     xcb_window_t window;
 
+   private:
     bool is_moved = false;
 };
 
@@ -99,11 +105,6 @@ class WineXdndProxy {
         Handle(Handle&&) noexcept;
         Handle& operator=(Handle&&) noexcept = default;
 
-        /**
-         * Handle X11 events for receiving XDND client messages.
-         */
-        void handle_x11_events() const noexcept;
-
        private:
         WineXdndProxy* proxy;
 
@@ -122,9 +123,6 @@ class WineXdndProxy {
      * in a COM object, we can only handle drag-and-drop coming form this
      * process.
      *
-     * The handle's `handle_x11_events()` method should be periodically called
-     * to pump the X11 events for handling XDND client messages.
-     *
      * This is sort of a singleton but not quite, as the `WineXdndProxy` is only
      * alive for as long as there are open editors in this process. This is done
      * to avoid opening too many X11 connections.
@@ -135,11 +133,28 @@ class WineXdndProxy {
     static WineXdndProxy::Handle get_handle();
 
     /**
-     * Handle X11 events for receiving XDND client messages.
+     * Initiate the XDDN protocol by taking ownership of the `XdndSelection`
+     * selection and setting up the event listeners.
      */
-    void handle_x11_events() const noexcept;
+    void begin_xdnd(
+        const boost::container::small_vector_base<std::string>& file_paths,
+        HWND tracker_window);
+
+    /**
+     * Release ownership of the selection stop listening for X11 events.
+     */
+    void end_xdnd();
 
    private:
+    /**
+     * From another thread, constantly poll the mouse position until
+     * `tracker_window` disappears, and then perform the drop if the mouse
+     * cursor was last positioned over an XDND aware window. This is a
+     * workaround for us not being able to grab the mouse cursor since Wine is
+     * already doing that.
+     */
+    void run_xdnd_loop();
+
     /**
      * We need a dedicated X11 connection for our proxy because we can have
      * multiple open editors in a single process (e.g. when using VST3 plugins
@@ -162,6 +177,30 @@ class WineXdndProxy {
         hook_handle;
 #pragma GCC diagnostic pop
 
+    /**
+     * The files that are currently being dragged.
+     */
+    boost::container::small_vector<std::string, 4> dragged_file_paths;
+
+    /**
+     * Wine's tracker window for tracking the drag-and-drop operation. Normally
+     * you would grab the mouse pointer when the drag-and-drop operation starts
+     * so you can track what windows you are hovering over, but we cannot do
+     * that because Wine is already doing just that. So instead we will
+     * periodically poll the mouse position from another thread, and we'll
+     * consider the disappearance of this window to mean that the drop has
+     * either succeeded or cancelled (depending on whether or not Escape is
+     * pressed).
+     */
+    HWND tracker_window;
+
+    /**
+     * We need to poll for mouse position changes from another thread, because
+     * when the drag-and-drop operation starts Wine will be blocking the GUI
+     * thread, so we cannot rely on the normal event loop.
+     */
+    Win32Thread xdnd_handler;
+
     // These are the atoms used for the XDND protocol, as described by
     // https://www.freedesktop.org/wiki/Specifications/XDND/#atomsandproperties
     xcb_atom_t xcb_xdnd_selection;