Timeline zoom ratio
It can be tiresome to use the Timeline's scrollbars when your animation layers have many frames.
Fortunately, it is possible to modify the Timeline's scale thanks to the 
button:
Click and hold on this button to change the Timeline's frame thumbnails' size (go left with your pen to shrink the thumbnails and go right to stretch them):
Right-clicking on this button will open the Auto Fit menu which may be used to automatically adjust the size of the thumnbails depening on the selected option (view the entire project, the clip, focus on the current layer, the current image, exposures, the current selection, keyframes or sound tracks...):
You can also use the ["Alt + Right-Click + Left / Right Movement"] key combination to shrink or stretch the thumbnails.