window layouter: fix glitch after window resize

This patch improves the transition from an interactive window geometry
change (dragging a window element) to the point where the resulting
new layout rules come into effect. During this short time, no resize
request must be issued because such a resize request would be based on
stale rules.

Fixes #3227

repos/gems/recipes/src/window_layouter/used_apis

@@ -4,3 +4,4 @@ input_session
 report_session
 nitpicker_session
 framebuffer_session
+timer_session

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

@@ -23,6 +23,7 @@
 #include <input_session/client.h>
 #include <input/event.h>
 #include <input/keycodes.h>
+#include <timer_session/connection.h>
 
 /* local includes */
 #include <window_list.h>
@@ -45,6 +46,13 @@ struct Window_layouter::Main : Operations,
 	Signal_handler<Main> _config_handler {
 		_env.ep(), *this, &Main::_handle_config };
 
+	Timer::Connection _drop_timer { _env };
+
+	bool _defer_layout_change = false;
+
+	Signal_handler<Main> _drop_timer_handler {
+		_env.ep(), *this, &Main::_handle_drop_timer };
+
 	Heap _heap { _env.ram(), _env.rm() };
 
 	Area _screen_size { };
@@ -126,7 +134,24 @@ struct Window_layouter::Main : Operations,
 	/**
 	 * Layout_rules::Change_handler interface
 	 */
-	void layout_rules_changed() override { _update_window_layout(); }
+	void layout_rules_changed() override
+	{
+		/*
+		 * When re-importing rules generated by the drop operation, issue an
+		 * updated resize request immediately instead of waiting for the
+		 * '_drop_timer_handler'. Clear '_defer_layout_change' before calling
+		 * '_update_window_layout' because '_gen_window_layout' evaluates
+		 * this flag.
+		 */
+		bool const issue_resize_request = _defer_layout_change;
+
+		_defer_layout_change = false;
+
+		_update_window_layout();
+
+		if (issue_resize_request)
+			_gen_resize_request();
+	}
 
 	/**
 	 * Window_list::Change_handler interface
@@ -216,6 +241,12 @@ struct Window_layouter::Main : Operations,
 		_gen_resize_request();
 	}
 
+	void _handle_drop_timer()
+	{
+		/* discharge '_defer_layout_change' */
+		layout_rules_changed();
+	}
+
 	void finalize_drag(Window_id id, Window::Element, Point, Point) override
 	{
 		/*
@@ -229,10 +260,15 @@ struct Window_layouter::Main : Operations,
 		_window_list.with_window(id, [&] (Window &window) {
 			window.finalize_drag_operation(); });
 
-		_gen_resize_request();
+		/*
+		 * Mask the generation of resize requests until the updated rules are
+		 * imported the next time. Discharge the masking after a timeout for
+		 * the case where rules fixed and not fed back to the layouter.
+		 */
+		_defer_layout_change = true;
+		_drop_timer.trigger_once(250*1000);
 	}
 
-
 	/**
 	 * Install handler for responding to hover changes
 	 */
@@ -340,6 +376,8 @@ struct Window_layouter::Main : Operations,
 		_nitpicker.mode_sigh(_mode_change_handler);
 		_handle_mode_change();
 
+		_drop_timer.sigh(_drop_timer_handler);
+
 		_hover.sigh(_hover_handler);
 		_decorator_margins_rom.sigh(_decorator_margins_handler);
 		_input.sigh(_input_handler);
@@ -360,6 +398,9 @@ struct Window_layouter::Main : Operations,
 
 void Window_layouter::Main::_gen_window_layout()
 {
+	if (_defer_layout_change)
+		return;
+
 	/* update hover and focus state of each window */
 	_window_list.for_each_window([&] (Window &window) {
 
@@ -377,6 +418,9 @@ void Window_layouter::Main::_gen_window_layout()
 
 void Window_layouter::Main::_gen_resize_request()
 {
+	if (_defer_layout_change)
+		return;
+
 	bool resize_needed = false;
 	_window_list.for_each_window([&] (Window const &window) {
 		if (window.resize_request_needed())