When you switch between MicroB windows it blinks white the second after switching. As I understand it re-reads rendered image from somewhere in RAM. But before the white blink I can see at least the part of the page I was looking at so clearing the page before refreshing is not necessary.
And as I use it very intensively it started to annoy me more and more being blinking like a stroboscope.
I tried reporting a bug but there is no Browser component for selection at https://bugs.maemo.org/enter_bug.cgi...fication=__all
It doesn't look like interface issue rather the engines', and the last is open, isn't it?

Yes, I too find this seemingly unnecessary refresh v. annoying. You're right that the UI is closed and the engine open. It might still be a UI bug - e.g. the UI for some reason telling the engine to refresh when the window gets a focus. I doubt this will be fixable in MicroB in CSSU so the only practical fix may be to switch to a different browser.