This patch is just an experiment in order to show a different possible behavior of the scrollbars. The incorporated change is not done in a way that could be accepted as-is - it is just hacked in. But could certainly be done better if we like this.
The issue this patch is illustrating is how the current hiding of the scrollbars also hides important details: your current position in the list and and estimation of how long the list is. It looks cleaner hiding the scrollbars but I'm not sure the current implementation is worth the loss of utility.
With this patch the scrollbars are always shown, if there is space to scroll. But does so at a reduced size and opacity. It is small enough to not get in the way or be distracting. But still large enough to indicate list position and length. Where the current scrollbars change opacity as your mouse gets closer to the edge, this version also increases the size of the scrollbars. This should make them easier to hit and use.
The following illustrates the changes in behavior. The top shows how it looks now. The left how Outliner might look when your mouse is not nearby, the right after the scrollbar appears.
The bottom shows how it looks after this patch is applied. The left how Outliner would always look with smaller scrollbar still indicating position in the list. The Right side shows the wider scrollbar while in use.