Click-through

When several windows overlap, the first click on one that is not currently active has two jobs competing for it: bring the window forward, and press whatever sits under the pointer. Click-through is the policy that lets one click do both. Without it the click is spent on the activation and the control never hears about it, so the reader clicks a second time on a button that was already under their finger.

The two policies are split down the middle of the desktop world. Windows generally clicks through, so the first click lands on the control. macOS generally does not: the activating click is swallowed, and only a small whitelist of controls is allowed to accept it, mostly the ones where a wrong press costs nothing, such as scrollbars, some toolbar buttons, and a background window’s own close box. The tension is honest on both sides. Click-through saves a click every single time, and costs a mistake the one time the reader meant only to bring the window forward and happened to put the pointer over Delete. That is why the exception list runs in the other direction as well: even in a click-through interface, destructive and irreversible controls tend to be excluded, because the click that reaches them was never aimed at them.

The same argument appears wherever one surface sits over another. A modal dialog’s scrim exists precisely to refuse click-through, so a click outside the dialog is spent on the dialog and never on the interface behind it. Nested inside a page, a floating panel that has just been opened faces the same choice about the click that opens it, and the answer is usually no for the same reason: the reader was aiming at the panel, not through it.

Two other uses of the phrase are worth keeping apart, since both are common. On the web, click-through often means a click passing through an overlay to what is underneath, which is pointer-events: none and a question about hit testing rather than about focus. In advertising it means a click that follows a link to its destination, as in click-through rate. Only the first sense is about the cost of activating a window, and it is the one that pairs with the focus models: click to focus and focus follows mouse decide when a surface becomes active, while click-through decides what the activating click is additionally allowed to do.