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There appears to be a problem with Qt 4.8 (I have tested 4.8.2). When you drag a ruler it is not erased until you stop dragging it. Therefore the ruler is quite painful to use. The same happens when moving the selection, so the situation is not acceptable.
I have not quite figured out what exactly behaves differently on Qt 4.8, but there is a quick workaround:
In layLayoutCanvas.cc replace that line (roughly around line 45):
#if defined (_WIN32) || defined (Q_WS_MAC)
#if defined (_WIN32) || defined (Q_WS_MAC) || QT_VERSION >= 0x040800
that probably has some performance impact when running KLayout over X11 connections, so I'll try to find a better fix.