A window-shopper is a person who looks at things in shop windows and displays without meaning to buy any of them. It is the person; the activity is window-shopping. The tone is usually light and friendly — walking around, looking at nice things, enjoying it, going home with nothing. A shop owner might say it with a small sigh, because window-shoppers take up space and spend no money, but even then it is not a strong insult. The word has stretched with the times. It still fits someone strolling past real shops, but people now use it for scrolling through an online store, filling a basket, and never buying, and 'window-shopper' works there too. You will also meet the picture used outside shopping: someone who looks at a job, a house, or a relationship without ever committing is called a window-shopper. That use is figurative and slightly critical. The usual spelling has a hyphen, though 'window shopper' as two words is common and fine.