Square block appears after app window is resized for Mac

Hello,
I m using openGL in my Qt project for mac, wherein I am using openGL for supporting panorama view. When I resize the app window, a block appears on my screen. I m running my app on mac mini 10.8.2 with Intel HD Graphics.
Also sometimes my buttons doesnt click and sometimes they do work properly.
I dont understand this random behavior of my application. Is it the Mac Mini problem or the graphics card, OR simply using openGL in QT on mac is not well supported at all?

I am attaching the file as error generation.