Add resistance when dragging tiled windows

Adds a config option <snapping><dragResistance> with default value 20.
This prevents tiled/maximized windows from being unintentionally untiled.

src/config/rcxml.c

@@ -971,6 +971,8 @@ entry(xmlNode *node, char *nodename, char *content)
 		} else {
 			wlr_log(WLR_ERROR, "ignoring invalid value for notifyClient");
 		}
+	} else if (!strcasecmp(nodename, "dragResistance.snapping")) {
+		rc.snap_drag_resistance = atoi(content);
 
 	/* <windowSwitcher show="" preview="" outlines="" /> */
 	} else if (!strcasecmp(nodename, "show.windowSwitcher")) {
@@ -1278,6 +1280,7 @@ rcxml_init(void)
 	rc.snap_overlay_delay_outer = 500;
 	rc.snap_top_maximize = true;
 	rc.snap_tiling_events_mode = LAB_TILING_EVENTS_ALWAYS;
+	rc.snap_drag_resistance = 20;
 
 	rc.window_switcher.show = true;
 	rc.window_switcher.preview = true;

src/input/cursor.c

@@ -232,9 +232,19 @@ request_set_primary_selection_notify(struct wl_listener *listener, void *data)
 static void
 process_cursor_move(struct server *server, uint32_t time)
 {
+	struct view *view = server->grabbed_view;
+
+	/*
+	 * Un-tile the view when interactive move is delayed and the distance
+	 * of cursor movement exceeds <snapping><dragResistance>.
+	 */
+	if (server->move_pending && !interactive_move_tiled_view_to(
+			server, server->grabbed_view, &server->grab_box)) {
+		return;
+	}
+
 	double dx = server->seat.cursor->x - server->grab_x;
 	double dy = server->seat.cursor->y - server->grab_y;
-	struct view *view = server->grabbed_view;
 
 	/* Move the grabbed view to the new position. */
 	dx += server->grab_box.x;

src/interactive.c

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0-only
+#include <assert.h>
 #include "edges.h"
 #include "input/keyboard.h"
 #include "labwc.h"
@@ -31,6 +32,38 @@ interactive_anchor_to_cursor(struct view *view, struct wlr_box *geometry)
 		view->current.height, geometry->height);
 }
 
+bool
+interactive_move_tiled_view_to(struct server *server, struct view *view,
+		struct wlr_box *geometry)
+{
+	assert(!view_is_floating(view));
+
+	int resistance = rc.snap_drag_resistance;
+
+	if (server->input_mode == LAB_INPUT_STATE_MOVE) {
+		/* When called from cursor motion handler */
+		assert(server->move_pending && server->grabbed_view == view);
+
+		double dx = server->seat.cursor->x - server->grab_x;
+		double dy = server->seat.cursor->y - server->grab_y;
+		if (dx * dx + dy * dy < resistance * resistance) {
+			return false;
+		}
+	} else {
+		/* When called from interactive_begin() */
+		if (resistance > 0) {
+			return false;
+		}
+	}
+
+	view_set_shade(view, false);
+	view_set_untiled(view);
+	view_restore_to(view, *geometry);
+	server->move_pending = false;
+
+	return true;
+}
+
 void
 interactive_begin(struct view *view, enum input_mode mode, uint32_t edges)
 {
@@ -83,9 +116,15 @@ interactive_begin(struct view *view, enum input_mode mode, uint32_t edges)
 				interactive_anchor_to_cursor(view, &geometry);
 			}
 
-			view_set_shade(view, false);
-			view_set_untiled(view);
-			view_restore_to(view, geometry);
+			/*
+			 * If <snapping><dragResistance> is non-zero, the
+			 * tiled/maximized view is un-tiled later in cursor
+			 * motion handler.
+			 */
+			if (!interactive_move_tiled_view_to(
+					server, view, &geometry)) {
+				server->move_pending = true;
+			}
 		} else {
 			/* Store natural geometry at start of move */
 			view_store_natural_geometry(view);
@@ -261,6 +300,7 @@ interactive_cancel(struct view *view)
 
 	view->server->input_mode = LAB_INPUT_STATE_PASSTHROUGH;
 	view->server->grabbed_view = NULL;
+	view->server->move_pending = false;
 
 	/* Update focus/cursor image */
 	cursor_update_focus(view->server);

src/xdg.c

@@ -94,6 +94,7 @@ do_late_positioning(struct view *view)
 	if (server->input_mode == LAB_INPUT_STATE_MOVE
 			&& view == server->grabbed_view) {
 		/* Keep view underneath cursor */
+		/* TODO: resistance is not considered */
 		interactive_anchor_to_cursor(view, &view->pending);
 		/* Update grab offsets */
 		server->grab_x = server->seat.cursor->x;