Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Guide on How To Customize The Safari Window
Customizing Safari is limited to its four bars, three of whom are at the top and one at the bottom. While customizing these bars, you can hide them, arrange them or customize what these bars show.
To Hide or Show The Bars: Select View. Look for Hide Bookmarks Bar, Status Bar, Toolbar or Tab Bar. Select the bar you want to hide. Once hidden the option changes to show. Next time, you want to show the bars, simply click on Show.
To Customize The Bars: Go to View and look for Customize Toolbar option. All the buttons on the toolbar now become drag-able. Drag the buttons out of the toolbar and click on Done.
To Rearrange Bookmark Bars or Toolbars: Ensure that the Safari window is active. Click on it to ensure this. Now press the Command Key and drag the buttons from the bookmarks. The only undrag-able icons are the Home and Arrow buttons.
There are also times when you want certain buttons to be visible when Safari is resized. For this go to the Button you want visible and double click on it. When prompted, select ‘Keep item Visible’.
As a piece of advice avoid downloading third party bars which clutter the top panel and make the actual viewing space lesser. You are better without them.