On Windows 10, each monitor may be configured with an arbitrary scaling factor, such as 125%, 150%, 200%, or even 300%. For instance, high-density laptop displays are typically set to a scaling of 200% by default. On Java 9 and 10, Swing handles this "HiDPI" scaling automatically, making the UI take up more pixels without looking smaller, and without any changes in the client application. This makes bitmap icons blurry or ragged, however.
To look good on HiDPI displays on Windows 10, the various icons that are part of NetBeans' window system must be made scalable to arbitrary resolutions. This includes, for instance, the "X" button that is used to close tabs, the "_" button that collapses a sidebar, or the ">>" button that shows hidden toolbar icons. These icons reside in the tabcontrol and openide.awt modules.