Spool fix_local_coordinates() call until release

This will cause the function to only be called on a `ConfigureNotify`
after all mouse buttons have been released. This prevents flickering
when dragging windows around.

src/wine-host/editor.cpp

@@ -469,6 +469,17 @@ void Editor::handle_x11_events() noexcept {
     //       function calls involving it will fail. All functions called from
     //       here should be able to handle that cleanly.
     try {
+        // HACK: See the docstrings on `should_fix_local_coordinates_` and
+        //       `fix_local_coordinates()`
+        if (should_fix_local_coordinates_ && !is_mouse_button_held()) {
+            logger_.log_editor_trace([&]() {
+                return "DEBUG: Performing spooled local coordinate fix";
+            });
+
+            fix_local_coordinates();
+            should_fix_local_coordinates_ = false;
+        }
+
         std::unique_ptr<xcb_generic_event_t> generic_event;
         while (generic_event.reset(xcb_poll_for_event(x11_connection_.get())),
                generic_event != nullptr) {
@@ -551,7 +562,21 @@ void Editor::handle_x11_events() noexcept {
                         event->window == parent_window_ ||
                         event->window == wrapper_window_.window_) {
                         if (!use_xembed_) {
-                            fix_local_coordinates();
+                            // NOTE: See the docstring on this field. This
+                            //       avoids flickering with some window manager
+                            //       and plugin combinations when dragging
+                            //       plugin windows around.
+                            if (is_mouse_button_held()) {
+                                logger_.log_editor_trace([&]() {
+                                    return "DEBUG: ConfigureNotify received "
+                                           "while mouse button is held down, "
+                                           "spooling the coordinate fix";
+                                });
+
+                                should_fix_local_coordinates_ = true;
+                            } else {
+                                fix_local_coordinates();
+                            }
                         }
                     }
                 } break;
@@ -959,6 +984,18 @@ std::optional<POINT> Editor::get_current_pointer_position() const noexcept {
         .y = query_pointer_reply->root_y + (win32_pos.top - x11_y_pos)};
 }
 
+bool Editor::is_mouse_button_held() const {
+    xcb_generic_error_t* error = nullptr;
+    const xcb_query_pointer_cookie_t pointer_query_cookie =
+        xcb_query_pointer(x11_connection_.get(), host_window_);
+    const std::unique_ptr<xcb_query_pointer_reply_t> pointer_query_reply(
+        xcb_query_pointer_reply(x11_connection_.get(), pointer_query_cookie,
+                                &error));
+    THROW_X11_ERROR(error);
+
+    return pointer_query_reply->mask != 0;
+}
+
 bool Editor::is_wine_window_active() const {
     if (!supports_ewmh_active_window()) {
         return false;

src/wine-host/editor.h

@@ -234,6 +234,11 @@ class Editor {
      * Lie to the Wine window about its coordinates on the screen for
      * reparenting without using XEmbed. See the comment at the top of the
      * implementation on why this is needed.
+     *
+     * One of the events that trigger this is `ConfigureNotify` messages. Some
+     * WMs may continuously send this message while dragging a window around. To
+     * avoid flickering, the main `handle_x11_events()` function will wait to
+     * call this function until the all mouse buttons have been released.
      */
     void fix_local_coordinates() const;
 
@@ -312,6 +317,12 @@ class Editor {
      */
     std::optional<POINT> get_current_pointer_position() const noexcept;
 
+    /**
+     * Checks whether any mouse button is held. Used to defer calling
+     * `fix_local_coordinates()` when dragging windows around.
+     */
+    bool is_mouse_button_held() const;
+
     /**
      * Returns `true` if the currently active window (as per
      * `_NET_ACTIVE_WINDOW`) contains `wine_window_`. If the window manager does
@@ -451,6 +462,15 @@ class Editor {
      */
     xcb_window_t host_window_;
 
+    /**
+     * Used to delay calling `fix_local_coordinates()` when dragging windows
+     * around with the mouse. Some WMs will continuously send `ConfigureNotify`
+     * messages when dragging windows around, and the `fix_local_coordinates()`
+     * function may cause the window to blink. This becomes a but jarring if it
+     * happens 60 times per second while dragging windows around.
+     */
+    bool should_fix_local_coordinates_ = false;
+
     /**
      * The atom corresponding to `_NET_ACTIVE_WINDOW`.
      */