Smart Checkout Option not being given

When I'm in a Git branch that has a dirty worktree and checkout another branch I am expecting to be shown a dialogue box giving me the option of selecting 'Smart Checkout' (as shown here - https://www.jetbrains.com/help/webstorm/2016.3/checking-out-switching-between-branches.html ).

Instead Webstorm changes to the new branch and shows the unchecked in changes in the new branch.

I'd like to be able to swap branches using 'Smart Checkout' but can't work out how to make this happen.