Win 10, GTX 1080
Blender 2.80, 2019-01-02 18:59, 11f2c65128dc

Normally with multiple input boxes one can click on the first one and then drag to select all of them. That way one can insert the same value in all of the boxes. It works for xyz boxes in "Scale" tab, but not in "Dimensions" tab. If you try to insert a new number in all xyz boxes at once, Blender will reset the "z" value to what was already in the "x" box, but will leave "y" intact.