I've spent entirely too much time worrying about tab placement recently, and being the rather insane person I am, I though other people might enjoy sharing in my misery. :)
Why am I worrying about tabs? Well, for one, I don't like how most tab bars work right now, and for two, I'm working on yet-another-IDE-to-change-the-world(TM) which will use tabs, and thus tab placement is now a very important topic.
First, I want to show two images that will help me get my ideas on this across,
Tabs to the top, and Tabs to the side
Both are of rdsEd, which is a wee IDE I've been cooking up the past few months using the Scintilla editing widget. Though, if I started with Scintilla, I wouldn't have spent the past few months trying to make my own with a Gtk TextView, some TextTags, and the skin o' me teeth. Plus I'm a bad coder.
But we're not here to talk about my many, many failings. No sir. We are here to listen to me go on and on with no point what so ever about tab placement. So, what I want to do here is make the case for the horizontal tab bar.
I want to first point out two things to notice about the tabs-on-top image. Firstly, it shows only 4 tabs, and secondly it would, given the chance, bleed the text of those tabs into a unreable mess of text that would require quite a strech of time to parse through the four visable tabs. (at least comparitively speaking) And this says nothing of the fact that we have eigth files open, thus four tabs hidden fiendishly from our sight.
Now, just for the fun of it, pick a random filename (any will do) and try to, with your eyes starting in the editor window's text entry area, mouse to it.
Now, try and do the same with the top-to-bottom tabs image.
Slightly less troublesome now, right? (Assuming you are right handed. If your left handed, you would likely want the tabs on the right side of the window, and would find them much easier to mouse to there. Or at least that's my theory...) Now, I want to take a moment to ask you a question: do you honestly click the tabs in, say, mozilla or gEdit? Or do you use the keyboard shortcuts to jump from tab to tab until you find the one you want? If so, do you ever find yourself becoming annoyed at it when it's hard to find the exact thing you want quickly?
Personally, I do. And I believe there is a reason for this.
When you move your eyes to the tabs, notice how they travel. When using the top-to-bottom tabs, you will most likely 1) look to the left, 2) find them resting in the center of the tabs, and thus 3) immediately be able to see where each name begins. On the one to the top, however, the first thing you have to do is locate where exactly the tabs are located. They have a large amount of space vertically, but when you quickly gaze to it the area you wish to mouse to will begin bleeding in with the toolbar. Your eyes and mouse will overshoot it. This would likely be worse if I weren't such a annoying person who sets GTK to use "Text under icons" in my toolbars. ;)
So the top-to-bottom, in my estimation, would be the better of the two. Even better, most of the wasted space is vertical, not horizontal. We have more vertical space to play with than horizontal by the nature of display sizes. We can use more of it more readily before we begin to grow too large for the screen.
The sole win for vertical tabs is that they are much more visually pleasing in a small window. The horizontal tabs have a much lower threshold against looking "crowded" than the vertical tabs. (Well, at least until you need to use the tabs. ;) Then the ball is back in the top-to-bottom tab bar's court, IMHO )
Simply put, it'd be better to not display the tabs at all than to place them from left-to-right. While placing the tabs horizontally we take a minor hit in vertical layout space, this is easy to recover by growing the window's vertical size as a whole and provides a much easier widget to deal with. Not that this stops anyone from using vertical tabs, but still...
Of course, this is all in my own head, and I'd be interested in hearing other opinions.