Correctly move focus from one output to a new one.

This patch aims to correctly handle moving focus <left|right|up|down>
between outputs.

For instance, if moving from one output to a new output at the left of
the current one, it should focus the right-most view/container on the
new output, and the opposite if moving from right to left. This should
happen regardless of the previously stored focus of the new output.

This also handles moving to a new output above or below the current one.

sway/layout.c

@@ -617,6 +617,50 @@ void arrange_windows(swayc_t *container, double width, double height) {
 	layout_log(&root_container, 0);
 }
 
+/**
+ * Get swayc in the direction of newly entered output.
+ */
+static swayc_t *get_swayc_in_output_direction(swayc_t *output, enum movement_direction dir) {
+	if (!output) {
+		return NULL;
+	}
+
+	swayc_t *ws = swayc_focus_by_type(output, C_WORKSPACE);
+	if (ws && ws->children->length > 0) {
+		switch (dir) {
+		case MOVE_LEFT:
+			// get most right child of new output
+			return ws->children->items[ws->children->length-1];
+		case MOVE_RIGHT:
+			// get most left child of new output
+			return ws->children->items[0];
+		case MOVE_UP:
+		case MOVE_DOWN:
+			{
+				swayc_t *focused_view = swayc_focus_by_type(ws, C_VIEW);
+				if (focused_view && focused_view->parent) {
+					swayc_t *parent = focused_view->parent;
+					if (parent->layout == L_VERT) {
+						if (dir == MOVE_UP) {
+							// get child furthest down on new output
+							return parent->children->items[parent->children->length-1];
+						} else if (dir == MOVE_DOWN) {
+							// get child furthest up on new output
+							return parent->children->items[0];
+						}
+					}
+					return focused_view;
+				}
+				break;
+			}
+		default:
+			break;
+		}
+	}
+
+	return output;
+}
+
 swayc_t *get_swayc_in_direction_under(swayc_t *container, enum movement_direction dir, swayc_t *limit) {
 	swayc_t *parent = container->parent;
 	if (dir == MOVE_PARENT) {
@@ -635,7 +679,8 @@ swayc_t *get_swayc_in_direction_under(swayc_t *container, enum movement_directio
 		sway_log(L_DEBUG, "Moving from fullscreen view, skipping to output");
 		container = swayc_parent_by_type(container, C_OUTPUT);
 		get_absolute_center_position(container, &abs_pos);
-		return swayc_adjacent_output(container, dir, &abs_pos, true);
+		swayc_t *output = swayc_adjacent_output(container, dir, &abs_pos, true);
+		return get_swayc_in_output_direction(output, dir);
 	}
 
 	if (container->type == C_WORKSPACE && container->fullscreen) {
@@ -649,7 +694,8 @@ swayc_t *get_swayc_in_direction_under(swayc_t *container, enum movement_directio
 		int diff = 0;
 		if (parent->type == C_ROOT) {
 			sway_log(L_DEBUG, "Moving between outputs");
-			return swayc_adjacent_output(container, dir, &abs_pos, true);
+			swayc_t *output = swayc_adjacent_output(container, dir, &abs_pos, true);
+			return get_swayc_in_output_direction(output, dir);
 		} else {
 			if (dir == MOVE_LEFT || dir == MOVE_RIGHT) {
 				if (parent->layout == L_HORIZ) {