Free Desktop mandates the user to remember which of two clipboards you are keeping selections in. If you switch between a terminal and browser, you might this UX jarring. This patch modifies st to work from one CLIPBOARD, the same as your browser.
This patch makes st set CLIPBOARD on selection.
st-clipboard-0.8.2.diff middle click pastes from CLIPBOARD.
You may want to replace selpaste with clippaste in your config.h to complete the affect.