Conveniently marked by the thin blue strip separating window contents and the title decoration is a realm where logic, physics, and good sense break down. This area only applies to windows with XCB-based applications running in a Wayland session, not native Wayland applications.

In practice it's a bit taller than the blue strip, but the blue strip is a good marker for where the issues are. It may take a few attempts to replicate the issues;

 - Clicking this area will sometimes pass the cursor through to the contents below. If the contents are another window, it will bring that window into the forefront.
 - On trackpads clicking this area will sometimes instead cause the cursor to switch into resizing mode. Dragging at this point will begin resizing the window from one of the top corners.