I didn't see official support answered this at all, but I found out this "unofficial answer" (well honestly I think Avast actually owns this website and do some promotion and unofficial answer"
http://www.getavast.net/support/compatibility-windows-10So there are 3 things you can try if this problem happens, may work may not work, may be permanent may be temporary and just like people talked about here any windows patch can let Avast screw up the taskbar again.
The website claims it is "Windows bug", while Avast is the only antivirus software has this problem, so Microsoft doesn't change programming code because of Avast then it is "Windows bug" while Avast doesn't want to change its software code may be because it is inside the core part so Avast doesn't want to change anything, you know too much work.
Yes some windows users have taskbar problem but it is not this case. I tried all those solution it wouldn't solve this, just because it is Avast causing this problem. The 3 steps above is totally different from the ways people try to solve taskbar problem without Avast, so it is two completely different issues even having similar problem looking.
The only permanent solution is uninstall Avast then install another one that you trust. I don't want to see my windows 10 just screw up again an again like how people said over here and I need to keep trying those 3 steps too see if I have luck. I don't want to waste hours again because of a free anti virus program just not really compatible with Windows 10 and claim it is Microsoft false.
I hope the 3 steps above help you if you really need to use Avast and you don't mind spending so much time and effort on this.