The conventional wisdom is to suggest trying the current version of Geany, though I doubt it will make any difference. The problem is like the desktop is not telling GTK its visible so it should re-draw.
I don't know Lubuntu at all, but if it has a setting for desktop acceleration, try turning it off. I have seen very slow screen redraws on Cinnamon with acceleration on, though that only affected the edit window and needed full screen as well.
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