Bugzilla

This is likely to be related to the "fling acceleration" feature that was added in bug 907123.

The behaviour of this can be configured using [a few prefs](https://searchfox.org/mozilla-central/rev/7ed8e2d3d1d7a1464ba42763a33fd2e60efcaedc/gfx/layers/apz/src/AsyncPanZoomController.cpp#252-270):

```
 * \li\b apz.fling_accel_interval_ms
 * The time that determines whether a second fling will be treated as
 * accelerated. If two flings are started within this interval, the second one
 * will be accelerated. Setting an interval of 0 means that acceleration will
 * be disabled.\n
 * Units: milliseconds
 *
 * \li\b apz.fling_accel_min_velocity
 * The minimum velocity of the second fling for it to be considered for fling
 * acceleration.
 * Units: screen pixels per milliseconds
 *
 * \li\b apz.fling_accel_base_mult
 * \li\b apz.fling_accel_supplemental_mult
 * When applying an acceleration on a fling, the new computed velocity is
 * (new_fling_velocity * base_mult) + (old_velocity * supplemental_mult).
 * The base_mult and supplemental_mult multiplier values are controlled by
 * these prefs. Note that "old_velocity" here is the initial velocity of the
 * previous fling _after_ acceleration was applied to it (if applicable).
```

Possibly we could pick better default for these prefs. (UX input on this would be helpful.) We could also take inspiration from how Chrome handles fling acceleration (some brief testing suggests it does it too, but possibly with different parameters).

This is likely to be related to the "fling acceleration" feature that was added in bug 907123.

The behaviour of this can be configured using [a few prefs](https://searchfox.org/mozilla-central/rev/7ed8e2d3d1d7a1464ba42763a33fd2e60efcaedc/gfx/layers/apz/src/AsyncPanZoomController.cpp#252-270):

```
 * \li\b apz.fling_accel_interval_ms
 * The time that determines whether a second fling will be treated as
 * accelerated. If two flings are started within this interval, the second one
 * will be accelerated. Setting an interval of 0 means that acceleration will
 * be disabled.\n
 * Units: milliseconds
 *
 * \li\b apz.fling_accel_min_velocity
 * The minimum velocity of the second fling for it to be considered for fling
 * acceleration.
 * Units: screen pixels per milliseconds
 *
 * \li\b apz.fling_accel_base_mult
 * \li\b apz.fling_accel_supplemental_mult
 * When applying an acceleration on a fling, the new computed velocity is
 * (new_fling_velocity * base_mult) + (old_velocity * supplemental_mult).
 * The base_mult and supplemental_mult multiplier values are controlled by
 * these prefs. Note that "old_velocity" here is the initial velocity of the
 * previous fling _after_ acceleration was applied to it (if applicable).
```

Possibly we could pick better defaults for these prefs. (UX input on this would be helpful.) We could also take inspiration from how Chrome handles fling acceleration (some brief testing suggests it does it too, but possibly with different parameters).