Paste a screenshot and hover to measure the space between edges.
Screenshot
Hover to measure and click to pin. With the image focused, arrow keys move the crosshair one image pixel (Shift for ten), Enter pins and Escape hides the crosshair. On a touch screen, drag on the image, then press Pin.Drag on the image to measure, then press Pin to keep the measurement. With a keyboard, arrow keys move the crosshair one image pixel (Shift for ten), Enter pins and Escape hides the crosshair.
Tolerance is how different a color can be from the pixel under the crosshair before it counts as an edge. Raise it for JPEGs and soft shadows, and lower it to catch faint borders.