By default it takes all available space on panel (if panel take all side of screen.) But can be option to take space only for buttons, not empty(it economies screen for other space-taking plugins, such as globalmenu)
This is not a problem with the tasklist, because the allocation code within the panel always tries to allocate the requested space. So what this mean is that the globalmenu plugin should either request enough space or it shouldn't be an expanding plugin. Making the tasklist not expand is not fixing the issue, because you will run into the same problem if you start more and more applications and the tasklist gets filled. So a proper fix would be better size request/allocation code in the globalmenu plugin. They use a gtk menu bar and that is one of the widgets which does not request a lot of space, else you cannot resize the window smaller then the menubar size. So they should override that in some way.
but the icon box in xfce4.6 not takes all available space and gives space to globalmenu. But with 4.8 we haw this: (screen)
Created attachment 3219 Screen with behaviour now
But must be window buttons only near notifications area (no empty space taken by it). Here is my request, for better understanding
You could indeed toggle the expansion of the iconbox in 4.6, but the problem with this was that when you opened enough applications, the button would the pushed offscreen, including the plugins after it. So there is no easy solution for this.
The tasklist doesn't take up extra space anymore, you need to add a separator that expands next to it. So this issue has been sort of fixed or the report is invalid.