window_layouter/decorator: drag-state corner cases

This patch complements the commit "nitpicker: defer hover changed while
dragging" with fixes of the window layouter and motif decorator.

- Handling of empty pointer model (after unhovering a decoration)
  instead of not updating it.

- Re-applying the hover model to the window layout when leaving the
  drag state. This addresses the corner case that the hover model
  changed during the drag operation (which is rightfully not handled
  while dragging).

- Letting the window layouter enter the drag state only if a dragged
  window is defined.

Issue #3973

repos/gems/src/app/decorator/main.cc

@@ -296,31 +296,16 @@ void Decorator::Main::_handle_gui_sync()
 
 	if (_window_layout_update_needed && _window_layout.valid()) {
 
-		try {
-			Xml_node xml(_window_layout.local_addr<char>(),
-			             _window_layout.size());
+		_window_stack.update_model(_window_layout.xml(), flush_window_stack_changes);
 
-			_window_stack.update_model(xml, flush_window_stack_changes);
+		model_updated = true;
 
-			model_updated = true;
-
-			/*
-			 * A decorator element might have appeared or disappeared under
-			 * the pointer.
-			 */
-			if (_pointer.valid())
-				update_hover_report(Xml_node(_pointer.local_addr<char>()),
-				                    _window_stack, _hover, _hover_reporter);
-
-		} catch (Xml_node::Invalid_syntax) {
-
-			/*
-			 * An error occured with processing the XML model. Flush the
-			 * internal representation with an empty window layout.
-			 */
-			_window_stack.update_model(Xml_node("<window_layout/>"),
-			                           flush_window_stack_changes);
-		}
+		/*
+		 * A decorator element might have appeared or disappeared under
+		 * the pointer.
+		 */
+		update_hover_report(_pointer.xml(),
+		                    _window_stack, _hover, _hover_reporter);
 
 		_window_layout_update_needed = false;
 	}
@@ -359,9 +344,7 @@ void Decorator::Main::_handle_pointer_update()
 {
 	_pointer.update();
 
-	if (_pointer.valid())
-		update_hover_report(Xml_node(_pointer.local_addr<char>()),
-		                    _window_stack, _hover, _hover_reporter);
+	update_hover_report(_pointer.xml(), _window_stack, _hover, _hover_reporter);
 }
 
 

repos/gems/src/app/window_layouter/main.cc

@@ -259,8 +259,10 @@ struct Window_layouter::Main : Operations,
 		 * Update window layout because highlighting may have changed after the
 		 * drag operation. E.g., if the window has not kept up with the
 		 * dragging of a resize handle, the resize handle is no longer hovered.
+		 *
+		 * The call of '_handle_hover' implicitly triggers '_gen_window_layout'.
 		 */
-		_gen_window_layout();
+		_handle_hover();
 
 		_drag_state = Drag_state::SETTLING;
 
@@ -308,10 +310,6 @@ struct Window_layouter::Main : Operations,
 	Signal_handler<Main> _decorator_margins_handler {
 		_env.ep(), *this, &Main::_handle_decorator_margins};
 
-
-	/**
-	 * Install handler for responding to user input
-	 */
 	void _handle_input()
 	{
 		while (_input.pending())
@@ -601,10 +599,11 @@ void Window_layouter::Main::_handle_hover()
 
 	/*
 	 * An exception may occur during the 'Xml_node' construction if the hover
-	 * model is malformed. Under this condition, we invalidate the hover state.
+	 * model lacks a window. Under this condition, we invalidate the hover
+	 * state.
 	 */
 	catch (...) {
-		
+
 		_user_state.reset_hover();
 
 		/*

repos/gems/src/app/window_layouter/user_state.h

@@ -282,21 +282,22 @@ void Window_layouter::User_state::_handle_event(Input::Event const &e,
 	}
 
 	/* detect end of drag operation */
-	if (e.release() && _key_cnt == 0 && _dragged_window_id.valid()) {
+	if (e.release() && _key_cnt == 0) {
 
 		_drag_state = false;
 
-		/*
-		 * Issue resize to 0x0 when releasing the the window closer
-		 */
-		if (_dragged_element == Window::Element::CLOSER) {
+		if (_dragged_window_id.valid()) {
 
-			if (_dragged_element == _hovered_element)
-				_operations.close(_dragged_window_id);
+			/*
+			 * Issue resize to 0x0 when releasing the the window closer
+			 */
+			if (_dragged_element == Window::Element::CLOSER)
+				if (_dragged_element == _hovered_element)
+					_operations.close(_dragged_window_id);
+
+			_operations.finalize_drag(_dragged_window_id, _dragged_element,
+			                          _pointer_clicked, _pointer_curr);
 		}
-
-		_operations.finalize_drag(_dragged_window_id, _dragged_element,
-		                          _pointer_clicked, _pointer_curr);
 	}
 
 	/* handle key sequences */