@xdydx @simontatham You mean X-style selection? The behaviour is configurable, but I thought it was that way by default on Linux, and https://the.earth.li/~sgtatham/putty/0.82/htmldoc/Chapter4.html#config-selection seems to agree.
@xdydx @simontatham But if you mean "select to copy to X's CLIPBOARD selection", then there's an "Auto-copy selected text" section on that page that describes how that can be configured.