cursor: allow scrolling tabs/stack on title border

This allows tabbed and stacked containers to be scrolled through when
the cursor is over the border of the title bar. The borders around the
other three edges of the contents should not be affected by this change.

sway/input/cursor.c

@@ -1092,6 +1092,8 @@ static void dispatch_cursor_axis(struct sway_cursor *cursor,
 	enum wlr_edges edge = cont ? find_edge(cont, cursor) : WLR_EDGE_NONE;
 	bool on_border = edge != WLR_EDGE_NONE;
 	bool on_titlebar = cont && !on_border && !surface;
+	bool on_titlebar_border = cont && on_border &&
+		cursor->cursor->y < cont->content_y;
 	bool on_contents = cont && !on_border && surface;
 	float scroll_factor =
 		(ic == NULL || ic->scroll_factor == FLT_MIN) ? 1.0f : ic->scroll_factor;
@@ -1117,7 +1119,7 @@ static void dispatch_cursor_axis(struct sway_cursor *cursor,
 	}
 
 	// Scrolling on a tabbed or stacked title bar (handled as press event)
-	if (!handled && on_titlebar) {
+	if (!handled && (on_titlebar || on_titlebar_border)) {
 		enum sway_container_layout layout = container_parent_layout(cont);
 		if (layout == L_TABBED || layout == L_STACKED) {
 			struct sway_node *tabcontainer = node_get_parent(node);