Broken: 2.77, 2.78,
Short description of error
Size of navigatiom frame of Flying mode highly depends on camera focus length (viewport) and camera start of clipping distance.
So if I want to work with small details and setting about 5—10 mm start.
Turning becomes extremely slow.
And with 20 cm cliping start and 35 mm frame becomes too small and navigation too fast.
Exact steps for others to reproduce the error
It happens after manipulating with focus length and clipping distance of viewport camera or even with camera object
Try with this blend file to adjust these values. Afterr few iterations navigation frame in flying mode can shrink to zero and it makes improssible to use this mode at all. This issue saves with file and do not appear in the new file or other scenes.