I put $20 that DOpus will get multiline tabs in version 12 or before. And it will be announced as a big new feature! 
Yes, but if I clicked on something in line 2, and then on something in line 1, I know visualy I can go now back to line 2. If I close something I know in few seconds again how to navigate. I'm using same thing in VStudio for years, and usually have 3-4 lines of tabs opened (even with custom tab coloring which TabsStudio allows - something that would also be very useful in dopus btw 
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With single line tabs you never know where you were if you use dropdown, everything is scrolling in both directions to keep selected tab visible. And then you have separator in dropdown to tell you which is hidden on left, and which is hidden on right. It's worse than multi line. There is no argument which will explain it's better.
Why did you implement dropdown anyway, then? 
And comparing dopus and browsers is not appropriate here, so let's not do that.
This is, actually, a great idea. I often end up having few 'log' folders opened on different servers and I have to look into tooltip where it points.
I think there are clearly cases where multi-line tabs is useful. I already work like this all the time in firefox and I know it works better than single-line (for me and obviously many others). It may not work for some people but that doesn't mean its isn't better and more efficient for others. Building up too many tabs and losing track of things is a problem can happen with single or multi-line tabs. If there are proper options to control tab-width, you can have multi-line tabs with immediate access and be able to read what is on each tab at the same time.
If mutli-line tabs won't get implemented because the developers don't like it, then that's how it will be. But with all of the options in Directory Opus, it seems like something that should be available like many other optional things many things which aren't necessary for many people. I think rather than simply saying over and over that people who want multi-line tabs are doing something wrong or inefficient, maybe it would be wise to consider that there are VALID reasons to want multi-line tabs and that in some cases, it will be a benefit. I agree with your sentiment that opening 100 tabs can get ridiculous but I can easily use multi-line tabs effectively with a reasonable amount of tabs and that's why I think it should be implemented in a good fashion. Besides, if someone wants to use 100 tabs in a ridiculous fashion, there may be a reason for that too but I'm more concerned with why I can't use multi-line tabs even for with more reasonable workflows. The most important point for me is that the tabs are always available and don't shuffle around much and with user-selectable tab widths, you can always be sure to read partially what's on the tabs.
Let's get back to a perfect example of exactly why I want multi-line tabs:
How do I display a reasonable 9 tabs on one side of a dual lister so I can see all of the tabs at the same time and they don't shrink down too much and I can still read the partial names on the tabs?
I've tried to explain how this can be done in a logical and efficient way with multi-line tabs. If someone can come up with a better way of doing that without multi-line tabs, I'll be quite happy but I don't think it will possible with a single line tabs because all the nine tabs squeeze too much together. I'm sure I'm not the only person running into this situation often enough.
One thing I know for sure, if GPSoftware ever does implement multi-line tabs, I have faith that it will be done in the best way possible. 
I just wish we could convince the developers that this really is worth implementing and even though it would be a lot of work, it would be great if it was available in the next major version approximately 2-3 years from now. Multi-line tabs and fine user control over tab width really is the only major feature I feel is missing in Directory Opus right now. All possible options regarding multi-line or single-line tabs and Tab Width options are already clearly laid out in TabMixPlus. It's a great blueprint to work from.
Just to add, I opened 3 tabs to my projects (I work on several projects like this each day):
Now, if this is too much tabs, then I guess my workflow sucks. It's only 4 of them.
But Leo made clear that we are doing something wrong and he's not interested in arguing with us
Thank you for the screenshot.
That is EXACTLY the kind of thing i have happening to me every day.
And I know from experience with TabMixPlus that the answer to this problem is multi-line tabs. It is a bit surprising there isn't at least some acknowledgement that mutli-line tabs would be beneficial in certain scenarios.
I didn't say four tabs was too many. I said 30.
That screenshot looks perfectly reasonable and fine, and it does not make a case for multiple lines of tabs. The tabs all fit in the screenshot, and the screenshot isn't even very wide.
(Remember you can turn on Preferences / Folder Tabs / Appearance / Reduce tab sizes to fit available space. And you can edit tab labels. And you can use the many alternatives to opening lots of tabs that I mentioned earlier in the thread.)