Startup actions are useful for customising TiddlyWiki according to environmental factors such as the screen size. For example, the following action widgets when placed in a tiddler tagged <<tag "$:/tags/StartupAction/Browser">> will cause the sidebar to be hidden by default when the screen width is less than 1000 pixels:
Note that global macros are not available within startup action tiddlers by default. If you need to access them then you'll need to explicitly include them: