from the testing I could conduct in the past few weeks, it is much better to have a 5 pixels spacing between thumbnails in the timeline (this should be combined with a color other than blue). However, what's lacking here is also an all-around "padding" instead of just spacing.

So the horizontal spacing should be 5 pixels (5 total), and the vertical spacing 4 pixels total (2 pixels top and 2 pixels bottom). The vertical spacing must be less than the horizontal spacing due to space constraints (less space available vertically, which would shrink the thumbnail).