By drag'n'drop a toolbar in the middle of the toolbars docking area, the existing toolbars on the right are shifted away, creating space for the new toolbar to dock. It may happen that the rightmost toolbar goes out of view and then it cannot be brought back to view. It is ticked off in the toolbars menu but it cannot be used unless the layout is deleted.
A better behaviour would be if the toolbars were filling the empty spaces when a toolbar is undocked.
Once I read your post, I don't think it is a problem, but when I'm testing now, I think it is really a problem. See the image below, when I drag the "Toolbar A" to the position before "Toolbar B", then the "Toolbar B " will move behind A in the same line, since A is too long, then B disappears. Even though I removed A, B is still out of view.
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