To zoom an area of the model window, we generally click the icon and then pick at two points defining the two corners of the box (window) for the zoomed area. Here is how you get the same sans clicking the corresponding icon.
Hold down the CTRL key, click and release (don't drag) LEFT button of your mouse at the first corner of the supposed zooming area; now move your cursor to the location of the other corner,click LMB again and let go the CTRL key.
Hold down the CTRL key, click and release (don't drag) LEFT button of your mouse at the first corner of the supposed zooming area; now move your cursor to the location of the other corner,click LMB again and let go the CTRL key.