In stand alone SlickEdit, it works only partially. If you left click in the same file, the selection remains, but if you switch to a different file (with either next-buffer or clicking the files tab) and left click, the selection is lost. This makes it very hard to copy or move code between files. To do it reliably, you must avoid using the mouse. Use the next-buffer key to switch files and then use the keyboard to move the cursor to your target. In this case, the selection will remain and you can move or copy the selection.
Note that in SlickEdit core, I want the selection to remain even if I switch files. I don't want to copy to the clipboard just to move or copy a selection between files.
Thanks for your help.