This video tutorial demonstrates how you can still use the mouse to navigate when using the Texture Application Tool in Valve Hammer Editor. In older versions of Hammer, you could click the camera tool when you had selected faces with the texture tool and then fly around the scene. Now, the moment you click the camera tool, all faces you had selected are deselected. Luckily, there is a keyboard shortcut (z) that will allow you to use the mouse for steering in the scene that does not deselect any faces you have selected.