Apple made a significant change to the tab bar in the latest version of Safari. Previously, a tab was as wide as it needed to be to display the page title. Once the page titles exceeded the width of the window, they would truncate evenly. This resulted in an empty space in the bar when there were only a few tabs with short titles. A plus (+) icon on the far right side would create a new tab, but I never used that, because double-clicking in the empty part of the bar would do the same thing.
Now, tabs expand to fill the width of the entire bar. If there is a single tab, it is the entire width of the window. Two tabs split it in half. As a result, there is never empty space in the bar, so to create a new tab requires moving the mouse all the way to the right side of the window to click that plus icon. Perhaps that’s not such a big deal, as you can also open a new tab by pressing Command + T, but I miss the double-click. That always seemed to be the most efficient method for me. I also think the extra-wide tabs look clunky, but that’s personal preference.
The other change to the bar is the addition of an icon to the right of the plus when there are multiple tabs. The icon represents a feature taken from Mobile Safari, where there aren’t tabs. It presents smaller representations of each tab and allows you to pan between them. I suppose it could be useful if you have a lot of tabs open, and you have to find a specific one by its visual appearance, rather than by its title. When this view is enacted, the tab bar goes away, and the plus icon moves up into the bookmarks bar. I haven’t found it to be a particularly useful feature so far, but it makes sense in maintaining some consistency between the desktop and mobile versions.