An axis-aligned rectangle, dragged one corner to the other.
The Rectangle draws a four-corner shape from a single drag. It is the fastest way to box an area of
a sheet.
Draw it: press at one corner, drag to the opposite corner, release. It is axis-aligned as
drawn; rotate it afterwards if it needs to sit at an angle.
Settings
Thickness: 0 to 20, default 1. Outline weight. Zero leaves the shape unstroked.
Colour: the outline.
Fill: a second colour channel on the same markup, so a shape can be outlined, filled, or both.
Opacity: applies to outline and fill together, which is what lets a filled box sit over linework without burying it.
Notes
A click with no drag places a shape only when the armed tool came from the Tool Chest and has a placement size saved on it, in which case it is born centred on the click. The standard toolbar Rectangle has no saved size, so a click draws nothing.