In Xubuntu, the default behaviour of window manager is to move the dragged window between workspaces. This behaviour becomes effective when having two or more workspaces (Xubuntu 16.04 uses only one workspace by default, thus ineffective at first).
To disable the behaviour, follow the steps.
Go to Settings > Window Manager.
In Advanced tab, under Wrap workspaces when reaching the screen edge, look for option "With a dragged window". Uncheck the option (leave the checkbox empty).
Close the window to finish.
The following screenshot shows before and after making changes to the option.
Now whenever a dragged window reaches to the screen edge, it will not switch to other workspaces. It will instead snap the dragged window to the half of screen (also the default behaviour).