My Inventory Glitched Out!

I was in the middle of fixing another bug, when my inventory wouldn't work properly.

I tried to put a block in my inventory, then it gave me a whole stack and I couldn't even place them in.

I'm not sure what triggered it. Maybe it was when I hit F8 or when I pressed both shifts but apart from that, I'm really not sure.

The only thing that works is playing with invert mouse. So now, I have to click and drag everything. It's really annoying!