desktop: keep windows within output on scale change

Fixes: #3195

include/desktop-state.h

@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_DESKTOP_STATE_H
+#define LABWC_DESKTOP_STATE_H
+#include <wayland-util.h>
+
+struct server;
+struct output;
+
+struct desktop_output {
+	struct output *output;
+	double scale;
+	struct wl_array views;
+	struct wl_list link;
+};
+
+/* Helpers to remember windows and scales for each output */
+void desktop_state_create(struct server *server, struct wl_list *desktop_outputs);
+void desktop_state_destroy(struct wl_list *desktop_outputs);
+
+/**
+ * desktop_state_adjust_on_output_scale_change() - adjust views on scale change
+ * @server: server
+ * @desktop_outputs: object list containing scale and array of views
+ *
+ * Put windows back on the pre-scale-change output and also adjust position/size
+ * to fit the usable area.
+ */
+void desktop_state_adjust_on_output_scale_change(struct server *server,
+	struct wl_list *desktop_outputs);
+
+#endif /* LABWC_DESKTOP_STATE_H */

src/desktop-state.c

@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#define _POSIX_C_SOURCE 200809L
+#include "desktop-state.h"
+#include <assert.h>
+#include <wlr/types/wlr_output.h>
+#include <wlr/util/log.h>
+#include "common/array.h"
+#include "common/border.h"
+#include "common/mem.h"
+#include "labwc.h"
+#include "output.h"
+#include "ssd.h"
+#include "view.h"
+
+void
+desktop_state_create(struct server *server, struct wl_list *desktop_outputs)
+{
+	struct output *output;
+	wl_list_for_each(output, &server->outputs, link) {
+		struct desktop_output *desktop_output = znew(*desktop_output);
+		desktop_output->output = output;
+		desktop_output->scale = output->wlr_output->scale;
+
+		/*
+		 * On output scale change, windows can move from one output to
+		 * another, so we have to remember where they were.
+		 */
+		struct view *view;
+		wl_array_init(&desktop_output->views);
+		wl_list_for_each(view, &server->views, link) {
+			if (view->output == output) {
+				array_add(&desktop_output->views, view);
+			}
+		}
+
+		wl_list_insert(desktop_outputs, &desktop_output->link);
+	}
+}
+
+void desktop_state_destroy(struct wl_list *desktop_outputs)
+{
+	struct desktop_output *desktop_output, *next;
+	wl_list_for_each_safe(desktop_output, next, desktop_outputs, link) {
+		wl_list_remove(&desktop_output->link);
+		wl_array_release(&desktop_output->views);
+		zfree(desktop_output);
+	}
+}
+
+static bool
+get_scale(double *scale, struct output *output, struct wl_list *desktop_outputs)
+{
+	struct desktop_output *desktop_output;
+	wl_list_for_each(desktop_output, desktop_outputs, link) {
+		if (desktop_output->output == output) {
+			*scale = desktop_output->scale;
+			return true;
+		}
+	}
+	assert(false && "output list mismatch");
+	return false;
+}
+
+static void
+fit_window_within_usable_area(struct view *view)
+{
+	struct wlr_box usable = output_usable_area_in_layout_coords(view->output);
+	struct border margin = ssd_get_margin(view->ssd);
+
+	/*
+	 * This is quite a simple approach.
+	 *
+	 * We prioritise keeping windows big because with a scale increase, it
+	 * is likely that users will want that. As such, we first move the
+	 * window to the top/left corner and then shrink it to the usable area
+	 * if needed.
+	 */
+	view_move(view, usable.x + margin.left, usable.y + margin.top);
+	view_constrain_size_to_that_of_usable_area(view);
+}
+
+void
+desktop_state_adjust_on_output_scale_change(struct server *server,
+		struct wl_list *desktop_outputs)
+{
+	struct desktop_output *desktop_output;
+	wl_list_for_each(desktop_output, desktop_outputs, link) {
+		struct view **view;
+		wl_array_for_each(view, &desktop_output->views) {
+			struct output *output = desktop_output->output;
+			double old_scale = 0.0;
+			if (!get_scale(&old_scale, output, desktop_outputs)) {
+				return;
+			}
+			/* Put window back on the pre-scale-change output */
+			if ((*view)->output != output) {
+				view_set_output(*view, output);
+			}
+			/* Adjust window position and size */
+			if (old_scale != (*view)->output->wlr_output->scale) {
+				fit_window_within_usable_area(*view);
+			}
+		}
+	}
+}

src/meson.build

@@ -3,6 +3,7 @@ labwc_sources = files(
   'buffer.c',
   'debug.c',
   'desktop.c',
+  'desktop-state.c',
   'dnd.c',
   'edges.c',
   'idle.c',

src/output.c

@@ -26,6 +26,7 @@
 #include "common/mem.h"
 #include "common/scene-helpers.h"
 #include "config/rcxml.h"
+#include "desktop-state.h"
 #include "labwc.h"
 #include "layers.h"
 #include "node.h"
@@ -791,6 +792,14 @@ handle_output_manager_apply(struct wl_listener *listener, void *data)
 
 	bool config_is_good = verify_output_config_v1(config);
 
+	/*
+	 * Save old desktop state, specifically windows and scales for each
+	 * output, so that window positions and sizes can be adjusted later.
+	 */
+	struct wl_list desktop_outputs;
+	wl_list_init(&desktop_outputs);
+	desktop_state_create(server, &desktop_outputs);
+
 	if (config_is_good && output_config_apply(server, config)) {
 		wlr_output_configuration_v1_send_succeeded(config);
 	} else {
@@ -803,6 +812,10 @@ handle_output_manager_apply(struct wl_listener *listener, void *data)
 			output->wlr_output->scale);
 	}
 
+	/* Adjust windows if output scale(s) changed */
+	desktop_state_adjust_on_output_scale_change(server, &desktop_outputs);
+	desktop_state_destroy(&desktop_outputs);
+
 	/* Re-set cursor image in case scale changed */
 	cursor_update_focus(server);
 	cursor_update_image(&server->seat);