Cursor: add workaround in pointer drag termination

Currently wlroots sends use the drag destroy event before sending the
wl_data_device.leave event to the client, which makes things a bit
awkward. My patch fixing this has been merged to wlroots master so we
can remove this when upgrading to wlroots 0.15, but until then this
workaround will fix the issue.

river/Seat.zig

@@ -84,6 +84,7 @@ status_trackers: std.SinglyLinkedList(SeatStatus) = .{},
 
 /// True if a pointer drag is currently in progress
 pointer_drag: bool = false,
+pointer_drag_idle_source: ?*wl.EventSource = null,
 
 request_set_selection: wl.Listener(*wlr.Seat.event.RequestSetSelection) =
     wl.Listener(*wlr.Seat.event.RequestSetSelection).init(handleRequestSetSelection),
@@ -128,6 +129,8 @@ pub fn deinit(self: *Self) void {
         self.focus_stack.remove(node);
         util.gpa.destroy(node);
     }
+
+    if (self.pointer_drag_idle_source) |idle_source| idle_source.remove();
 }
 
 /// Set the current focus. If a visible view is passed it will be focused.
@@ -476,6 +479,7 @@ fn handleStartDrag(listener: *wl.Listener(*wlr.Drag), wlr_drag: *wlr.Drag) void
     if (wlr_drag.icon) |wlr_drag_icon| {
         const node = util.gpa.create(std.SinglyLinkedList(DragIcon).Node) catch {
             log.crit("out of memory", .{});
+            wlr_drag.seat_client.client.postNoMemory();
             return;
         };
         node.data.init(self, wlr_drag_icon);
@@ -486,8 +490,25 @@ fn handleStartDrag(listener: *wl.Listener(*wlr.Drag), wlr_drag: *wlr.Drag) void
 
 fn handlePointerDragDestroy(listener: *wl.Listener(*wlr.Drag), wlr_drag: *wlr.Drag) void {
     const self = @fieldParentPtr(Self, "pointer_drag_destroy", listener);
-    self.pointer_drag = false;
     self.pointer_drag_destroy.link.remove();
+
+    // TODO(wlroots): wlroots 0.14 doesn't send the wl_data_device.leave event
+    // until after this signal has been emitted. Triggering a wl_pointer.enter
+    // before the wl_data_device.leave breaks clients, so use an idle event
+    // source as a workaround. This has been fixed on the wlroots master branch
+    // in commit c9ba9e82.
+    const event_loop = server.wl_server.getEventLoop();
+    assert(self.pointer_drag_idle_source == null);
+    self.pointer_drag_idle_source = event_loop.addIdle(*Self, finishPointerDragDestroy, self) catch {
+        log.crit("out of memory", .{});
+        wlr_drag.seat_client.client.postNoMemory();
+        return;
+    };
+}
+
+fn finishPointerDragDestroy(self: *Self) callconv(.C) void {
+    self.pointer_drag_idle_source = null;
+    self.pointer_drag = false;
     self.cursor.checkFocusFollowsCursor();
     self.cursor.updateState();
 }