Drag (left mouse button) slowly to display the viewport.
As you begin to drag, a rectangle with two
diagonals appears and continues to grow as long as you continue to
drag. This rectangle represents the viewing window of the future
view.
Before
After
Continue dragging to move around, resize and reposition
the viewport.
The viewport is then shaped like a pyramid: your
eyepoint is located at the vertex of the pyramid. You can resize
the viewport by dragging the middle mouse button.
Before
After
Release the button.
You now see what is targeted inside the viewport.
You can also press and hold down both Shift then the middle
mouse button for a quicker result.