The selection is lost when I use keyboard to select text

When I use keyboard (Shift + arrow key) to select my text range, and then input gist description, press Enter key,  my selection range is missing, it is not selected instead now. If I continue to press Enter key to sumbit, the result is the whole file will be pushed to Github Gist. But if I use mouse to select text range, it works very well like I expect. Any help will be appropriate.
