Particularly noticeable with the GTK3 version (I haven't tested other versions, though), the lack of border separating the top toolbars area from the contents makes the UI look a little bit "unintentional", in the sense that it does not look as polished as it could be, and it also makes it a little bit hard for the eye to clearly make out the separation.
Unlike the other toolbars, the Tabbed toolbar has, possibly by accident (because it's a border all around the toolbar) a very subtle very-light-grey border, but the other toolbars (and even this one) would very much benefit from a 50% opacity hard 1px border line. When running a light theme, 50% of pure black gives you gray and it looks fine; I haven't tested with a dark theme but it's either going to be fine or require maybe a slightly darker border (75% opacity?), though typically dark themes won't be affected as much by the lack of border as the page contents are typically black-on-white so the distinction is pretty clear.
Created attachment 170605 [details]
top toolbars border styles comparison - screenshot of the current state
Created attachment 170606 [details]
top toolbars border styles - comparison - with 1px borders
Created attachment 170607 [details]
top toolbars border styles - comparison - with shadow gradients
In an ideal world, I'd go one step further and suggest that there is a smooth subtle (25% opacity at the top) shadow gradient going from the bottom of the top toolbar area towards the top of the page contents area, but 1) this is probably much more complicated to implement, and maybe you'd suggest there be a standalone feature request about that 2) there might be performance implications (?) that the 1px border wouldn't have, when painting this on the document canvas.
*** Bug 124667 has been marked as a duplicate of this bug. ***
*** Bug 133691 has been marked as a duplicate of this bug. ***
would be very great if this can be implemented. at least in gtk3 it's very difficult to separate toolbar and content space. For example Thunderbird and Firefox make the separation very well.
*** Bug 140266 has been marked as a duplicate of this bug. ***
(In reply to Buovjaga from comment #7)
> *** Bug 140266 has been marked as a duplicate of this bug. ***
Bug 140266 was about toolbars docked to the bottom, missing a top border.