Improve behaviour of drag-n-drop during window manager operations

* src/xterm.c (x_dnd_begin_drag_and_drop): Select for some
events on the root window.
(x_dnd_update_state): New function.
(handle_one_xevent): Call that function when we get some events
from the root window substructure or the window manager.
(x_term_init): New atom `_NET_CLIENT_LIST_STACKING'.
* src/xterm.h (struct x_display_info): New atom
`_NET_CLIENT_LIST_STACKING'.

2 files changed, 108 insertions, 1 deletions

diff --git a/src/xterm.c b/src/xterm.c
index 4f8accbda3a..3e888407b3b 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -1149,6 +1149,7 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
 #ifdef HAVE_X_I18N
   XIC ic = FRAME_XIC (f);
 #endif
+  XWindowAttributes root_window_attrs;
 
   struct input_event hold_quit;
   char *atom_name;
@@ -1187,7 +1188,21 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
   current_count = 0;
 #endif
 
+  /* Now select for SubstructureNotifyMask and PropertyNotifyMask on
+     the root window, so we can get notified when window stacking
+     changes, a common operation during drag-and-drop.  */
+
   block_input ();
+  XGetWindowAttributes (FRAME_X_DISPLAY (f),
+                        FRAME_DISPLAY_INFO (f)->root_window,
+                        &root_window_attrs);
+
+  XSelectInput (FRAME_X_DISPLAY (f),
+                FRAME_DISPLAY_INFO (f)->root_window,
+                root_window_attrs.your_event_mask
+                | SubstructureNotifyMask
+                | PropertyChangeMask);
+
 #ifdef HAVE_X_I18N
   /* Make sure no events get filtered when XInput 2 is enabled.
      Otherwise, the ibus XIM server gets very confused.  */
@@ -1230,6 +1245,10 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
 #ifdef HAVE_X_I18N
           FRAME_XIC (f) = ic;
 #endif
+          /* Restore the old event mask.  */
+          XSelectInput (FRAME_X_DISPLAY (f),
+                        FRAME_DISPLAY_INFO (f)->root_window,
+                        root_window_attrs.your_event_mask);
           unblock_input ();
           quit ();
         }
@@ -1243,6 +1262,11 @@ x_dnd_begin_drag_and_drop (struct frame *f, Time time, Atom xaction,
   current_hold_quit = NULL;
 #endif
 
+  /* Restore the old event mask.  */
+  XSelectInput (FRAME_X_DISPLAY (f),
+                FRAME_DISPLAY_INFO (f)->root_window,
+                root_window_attrs.your_event_mask);
+
   unblock_input ();
 
   if (x_dnd_return_frame == 3)
@@ -10566,6 +10590,74 @@ mouse_or_wdesc_frame (struct x_display_info *dpyinfo, int wdesc)
     }
 }
 
+/* Get the window underneath the pointer, see if it moved, and update
+   the DND state accordingly.  */
+static void
+x_dnd_update_state (struct x_display_info *dpyinfo)
+{
+  int root_x, root_y, dummy_x, dummy_y, target_proto;
+  unsigned int dummy_mask;
+  Window dummy, dummy_child, target;
+
+  puts ("us");
+
+  if (XQueryPointer (dpyinfo->display,
+                     dpyinfo->root_window,
+                     &dummy, &dummy_child,
+                     &root_x, &root_y,
+                     &dummy_x, &dummy_y,
+                     &dummy_mask))
+    {
+      target = x_dnd_get_target_window (dpyinfo, root_x,
+                                        root_y, &target_proto);
+
+      if (target != x_dnd_last_seen_window)
+        {
+          if (x_dnd_last_seen_window != None
+              && x_dnd_last_protocol_version != -1
+              && x_dnd_last_seen_window != FRAME_X_WINDOW (x_dnd_frame))
+            x_dnd_send_leave (x_dnd_frame, x_dnd_last_seen_window);
+
+          if (x_dnd_last_seen_window == FRAME_X_WINDOW (x_dnd_frame)
+              && x_dnd_return_frame == 1)
+            x_dnd_return_frame = 2;
+
+          if (x_dnd_return_frame == 2
+              && x_any_window_to_frame (dpyinfo, target))
+            {
+              x_dnd_in_progress = false;
+              x_dnd_return_frame_object
+                = x_any_window_to_frame (dpyinfo, target);
+              x_dnd_return_frame = 3;
+            }
+
+          x_dnd_last_seen_window = target;
+          x_dnd_last_protocol_version = target_proto;
+
+          if (target != None && x_dnd_last_protocol_version != -1)
+            x_dnd_send_enter (x_dnd_frame, target,
+                              x_dnd_last_protocol_version);
+        }
+
+      if (x_dnd_last_protocol_version != -1 && target != None)
+        x_dnd_send_position (x_dnd_frame, target,
+                             x_dnd_last_protocol_version,
+                             root_x, root_y, x_dnd_selection_timestamp,
+                             dpyinfo->Xatom_XdndActionCopy);
+    }
+  /* The pointer moved out of the screen.  */
+  else if (x_dnd_last_protocol_version)
+    {
+      if (x_dnd_last_seen_window != None
+          && x_dnd_last_protocol_version != -1)
+        x_dnd_send_leave (x_dnd_frame,
+                          x_dnd_last_seen_window);
+
+      x_dnd_in_progress = false;
+      x_dnd_frame = NULL;
+    }
+}
+
 /* Handles the XEvent EVENT on display DPYINFO.
 
    *FINISH is X_EVENT_GOTO_OUT if caller should stop reading events.
@@ -11006,6 +11098,12 @@ handle_one_xevent (struct x_display_info *dpyinfo,
             }
         }
 
+      if (event->xproperty.window == dpyinfo->root_window
+          && (event->xproperty.atom == dpyinfo->Xatom_net_client_list_stacking
+              || event->xproperty.atom == dpyinfo->Xatom_net_current_desktop)
+          && x_dnd_in_progress)
+        x_dnd_update_state (dpyinfo);
+
       x_handle_property_notify (&event->xproperty);
       xft_settings_event (dpyinfo, event);
       goto OTHER;
@@ -11219,6 +11317,9 @@ handle_one_xevent (struct x_display_info *dpyinfo,
       if (xg_is_menu_window (dpyinfo->display, event->xmap.window))
         popup_activated_flag = 1;
 #endif
+
+      if (x_dnd_in_progress)
+        x_dnd_update_state (dpyinfo);
       /* We use x_top_window_to_frame because map events can
          come for sub-windows and they don't mean that the
          frame is visible.  */
@@ -12181,6 +12282,9 @@ handle_one_xevent (struct x_display_info *dpyinfo,
 #endif
 
         }
+
+      if (x_dnd_in_progress)
+        x_dnd_update_state (dpyinfo);
       goto OTHER;
 
     case ButtonRelease:
@@ -12460,6 +12564,8 @@ handle_one_xevent (struct x_display_info *dpyinfo,
       break;
 
     case CirculateNotify:
+      if (x_dnd_in_progress)
+        x_dnd_update_state (dpyinfo);
       goto OTHER;
 
     case CirculateRequest:
@@ -18801,6 +18907,7 @@ x_term_init (Lisp_Object display_name, char *xrm_option, char *resource_name)
       ATOM_REFS_INIT ("_NET_WM_FRAME_DRAWN", Xatom_net_wm_frame_drawn)
       ATOM_REFS_INIT ("_NET_WM_USER_TIME", Xatom_net_wm_user_time)
       ATOM_REFS_INIT ("_NET_WM_USER_TIME_WINDOW", Xatom_net_wm_user_time_window)
+      ATOM_REFS_INIT ("_NET_CLIENT_LIST_STACKING", Xatom_net_client_list_stacking)
       /* Session management */
       ATOM_REFS_INIT ("SM_CLIENT_ID", Xatom_SM_CLIENT_ID)
       ATOM_REFS_INIT ("_XSETTINGS_SETTINGS", Xatom_xsettings_prop)
diff --git a/src/xterm.h b/src/xterm.h
index ed612c3c1e9..2a11f87e167 100644
--- a/src/xterm.h
+++ b/src/xterm.h
@@ -550,7 +550,7 @@ struct x_display_info
     Xatom_net_workarea, Xatom_net_wm_opaque_region, Xatom_net_wm_ping,
     Xatom_net_wm_sync_request, Xatom_net_wm_sync_request_counter,
     Xatom_net_wm_frame_drawn, Xatom_net_wm_user_time,
-    Xatom_net_wm_user_time_window;
+    Xatom_net_wm_user_time_window, Xatom_net_client_list_stacking;
 
   /* XSettings atoms and windows.  */
   Atom Xatom_xsettings_sel, Xatom_xsettings_prop, Xatom_xsettings_mgr;