How can I get safari to show me the pixel dimensions of the browser window, without an extension? I have once seen safari showing me the pixel dimensions of the browser window, even when I resized it dragging left or right, it showed how many pixels in the x axis and how many pixels in the y axis.

But now when I resize the window it doesn't show the number of pixels of any current size.

How can I get the resolution / number of pixels in x and y, to show when I resize the safari browser window(as I've seen it do before)?

note- (was formerly - mistakenly - titled as safari has stopped showing me the resolution when resizing a window)
You have to have the element inspector open.  So with element inspector open, then, In Safari, when you hover over an element, it shows the pixels. 

And you can get it to show the pixels while you resize a window, to do that you click the crosshair icon in the inspector, then hover over the background so it shows the pixels, then move the mouse across so it still shows the pixels, and resize the window and it will update the pixel dimensions.  

Note that in Chrome, if you have the inspector open, and resize, then it shows you the pixel dimensions while you resize.