Notice that only that one corner changed. The Corners dialog box allows you to edit the corner
type and radius, but it also has an extra option called Rounding for setting absolute versus
relative rounding. Absolute means the rounded corner is exactly the radius value. Relative
makes the radius value based on the angle of the corner point.
Click the bottom-corner widget ( ) to select just that one corner widget. See the first part of
the following figure.
Drag the bottom-corner widget ( ) away from the center of the shape, until you see 0 in the
measurement label. You will need to drag past the corner point.
Select the Selection tool ( ) in the Tools panel. Click the Fill color box in the Properties panel,
and make sure that the Swatches option ( ) is selected in the panel that appears. Select a
darker red color to fill the rectangle with red.
Note
You can press the Escape key to hide the panel before moving on.