How to bisect a selection with GIMP?

Question

How can I cut a rectangular selection exactly in half, so that a 128-pixel long selection becomes 64 pixels long?

I have this:

I want to select just the top half of this box (so I can bucket fill it with brown), then select just the bottom half and bucket fill it with green.

The end result should be a box that is split perfectly between two colors.

Answer 1

Bucket-fill the whole selection with the colour you want left in the bottom half, then:

• pick the Rectangle Select tool again, click on the centre of the existing selection (to re-activate the tool on that same selection);
• in the Tool Options dialogue Size field (y-dimension), append "/2" to the value currently displayed there (meaning divide by 2), and click on the centre of the selection again - the selection should change to just the upper half;
• bucket-fill the new selection.