Reproduction steps:
1. Open the attached project "animator_transitionSlider_case871160";
2. Open the scene "test";
3. Go to the Animator window;
4. Click on the "Walker-Collapse" or "Walker_Sitting";
5. In the Inspector window click Transitions;
6. Click on the Transition slider and drag it across the timescale backwards and forwards;

Actual result: the slider position trails behind or in front of the mouse cursor (actual_transitionSlider.gif).
Expected result: the slider position should be in line with the mouse cursor (expected_transitionSlider.gif).

Reproduced with: 5.4.0b1, 5.4.2f1, 5.5.0f3, 5.6.0b4
Not reproducible: 5.3.6p4, 5.4.0a2
Regression introduced in 5.4.0b1