This is not a PHPStorm problem. By default Gnome comes with the <Alt> Modifier to the clic so you can press that key and with the mouse clic holded move the window. But I think that it’s unnecesary and generates a problem with PHPStorm multiple cursors, now if I press Alt and clic where I need a new cursor it does nothing.
In order to fix this just change the key as the modifier.
1. Install dconf editor
sudo apt-get install dconf-editor
2. Run it with Unity
3. Search the following tree config.
org -> gnome -> desktop -> wm -> preferences
4. Change the value of the key “mouse-button-modifier” in my case I change it to <Super> don’t leave it none because it will set the mouse to move the windows always and it’s pretty stressful.