eirk
I dont think I agree its nescessary. you can try Firefox as another example of how its done without
theres not even the little line seperating the tabs. and it isnt less intuative to me i can move stuff around.
Like i follow what you are saying that it makes a difference when its on the same horizontal line. but to me it seems like unescessary visual glutter, I think people will get it without
but as i also posted above it can be done both ways, with a permanent border, or one that only activates on hover.
So if people really thinks its an issue that they are unsure the tabs can be dragged or whatever, you can have the permanent border.
or you can make a small line like firefox have at the begnining
In my sketches theese things also only aplies to "color mode" (as a seperate feature to compact tabs) where flat design kicks in, and there the goal is imersion. so as little as possible disturbing the website. theres nothing practical in having the toolbar visually merge with the website. so if you prefer functionality and clear visual seperation then i dont think color mode or compact tabs for that matter is for you.
But if you want Compact tabs without color mode this is how it would look:
