This is not a PHPStorm problem. By default Gnome comes with the 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 unnecessary 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” to <Super>
If you leave it none it will set the mouse to move the windows always, and it’s pretty stressful that way.
Now you can leave Alt key pressed and clic through the text to generate multiple cursors on PHPStorm