Here are the options that may affect this problem (Customize – Tab):
Tab style: Tab
Icons: Document type icons
When Not Fit: None
Mouse Options -> Close Button: Right Side of Window
Now, I just open many documents, so there are too many tab headers to fit the screen and two scroll buttons appear. Then by clicking the left scroll button I scroll the tabs to the right (now some of them are “under” the scroll buttons), and then (by clicking the right button) – back to the left. And when the tabs are moving outside (from under the scroll buttons), they have the picture of those buttons on them, as shown on the screenshot in my previous post. Of course, this picture is not permanent – it disapplears when I move the mouse cursor over the tab.