How do you programmatically resize and move windows with the Windows API?

Question

How do you programmatically resize and move windows with Windows API?

The scenario is: I want to vertically tile two windows (80%/20% width proportions).

Answer 1

Use SetWindowPos() in the Windows API.

Given a HWND, you can change the z-order, position (upper left), size, as well as numerous other flags (like showing or hiding, making it the active window or not, etc).