This is simply what the GTK toolkit does if the pane is too small to fit its contents, if it won't fit it won't fit, GTK has to do something, so it cuts it off.

If you make the pane wider it won't be cut off. There is a drag handle between the sidebar and the editor, but whatever theme your disto is using seems to make it invisible :( but possibly the cursor changes when its over it.

Oh you're right moving the sidebar makes it visible, thanks!