Floodfill
The floodfill tool fills a chosen area with the provided amount of blocks to place. The Active Block is the block used to fill an area.
The tool fills an area by equally spreading across and down from the selected starting position. The amount of blocks used can be set using the Limit Slider in the Tool Options, and a visualisation of the floodfill is shown when hovering over the Main View Window.
Using right-click will initiate a floodfill at the position of your cursor.