It seems that the Safari address bar width is locked to a maximum size that may make some URL impossible to read, especially if the domain part is longer, like in many intranets, or when the the of the URL is longer and when it contains essential information near the end.
I fount no way to change the width of the bar using Customize Toolbar option.
I am using Safari 11 on macOS 10.13 (high-sierra) but I am pretty sure that the same problem would apply for macOS 10.12 (Sierra).
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