I found the bug. The problem is triggered by changing the firefox window. This calls a function that is supposed to change the size of the history plot and it works well when the history plots are visible but not if the history plots are hidden in a javascript tab (not another firefox tab).

Is there a clean way to resize the history plot if the parent div changes size?? The offending code is
mhist[i].mhg = new MhistoryGraph(mhist[i]);
mhist[i].mhg.initializePanel(i);
mhist[i].mhg.resize();
mhist[i].resize = function () {
   mhis.mhg.resize();
};