Hello @rwurzer!
Thank you very much for taking your time to report us that problem! It really looks like the component could get a good review to become better visually-appealing.
The example in question is:
TabPanel > Basic > Advanced Tabs
I see that glitch is triggered using the blue or gray theme if we click the tabs around, making it scroll. By pulling the tab via the arrow scrollers this does not happen. Resizing the window horizontally also triggers the issue.
The glitch happens also in other themes, but at least on the default theme, Triton, it is very hard to notice, as the inactive tabs' background is not outlined.
It looks like the right scroller button should occupy that space, rather than having it "half-behind" the dropdown menu button.
There's already an issue bound to that feature. In fact, this is a feature that has not been working in Ext JS for some time now (since around 2015) and was fixed on Ext.NET alone. This means as much as we can fix it, Sencha can come in new versions with a "backpush" breaking it again without further notice, simply because they are not supporting the feature.
Anyway, we'll take a look on that, and post an update here as soon as we get that glitch fixed.
The github issue in question is
#552.