Disagree, if a user wants to make it narrower so they can see the far right of lines they are editing they should not be prevented from doing so by the software. The "slide off the screen left" is the method the GTK toolkit provides for handling a width that is too narrow. Users who do not want it hidden can simply not make it so narrow. If there is anbetter method of handling it provided by GTK then "somebody" can make a PR.
The being too small on first startup should be improved in latest git version (or soon).