happens to me. Alot. and i also know how to fix it (i think).
this problem occurs when you turn on shaders and when you switch windows from toribash using alt-tab or just clicking outside the window.
to solve this, click on the tab labled toribash on the bottom of the screen with all ur aplications on. and then click it again. so it switches from toribash (half blank) to random document, to toribash (alls good). basicly: click on torbash icon 2 times.
thats just an easier way (for me anyways)