Are you seeing this in Feb release, as I remember this wasn't like this in earlier releases -
Looks like a UI design bug to me, no logic I could think of having such a thin scroll bar.
I had faced problem in bar/combo chart in situations when data volume was too high and the bottom minichart which is used for scrolling purpose was showing very thin line. I contacted Qlik Support, as per them it is as designed and I should not be showing high volume data in Bar/combo.
You may like to contact Qlik support if you don't find any work around here.