Pretty much most of the Sony Xperia Z flagship range of phones have been slippery phones to handle, not helped by the use of front and back glass panels. However, Sony appears to have improved handling in the latest flagship, the Xperia Z4 (Z3 Plus).
The frame of the Xperia Z4/Z3+ includes a very small groove/lip on the edges, which apparently markedly improves grip. This includes better grip both in the hand and even on surfaces. The short video demonstrates this, comparing the Xperia Z4/Z3+ sliding on an incline versus the Xperia Z3. Looks like Sony has made a lot of small changes to the Xperia Z4/Z3+ to improve it over its predecessor.
The Xperia Z4/Z3+ edge (top) has a slightly raised edge versus the Xperia Z3 (bottom)