3d plots using the default renderer soemtimes don't show

[jirilebl]

Sometimes, and I don't know at all how to exactly trigger it, a 3d plot just doesn't show. This happens when the output was there in a worksheet that was closed and I open it. I can try to rerun the code but the output is still not there. The output frame is just totally empty white space. If I change anything trivial about the code and rerun it, the plot it shows, if I change it back and rerun, it gives empty space again. This is in the standard jupyter notebook with the latest sage kernel and I've reproduced it with both Firefox and Chrome. Then a few minutes later it may start working again even for the original parameters. The latest one it was not working for a while was

var('x y')
f(x,y)=(xy)/(x^2+y^2)
gr=plot3d(f(x,y), (x, -1, 1), (y, -1, 1), opacity=0.8, plot_points=150)
gr=gr+parametric_plot3d((x,0,f(x,0)),(x,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((0,y,f(0,y)),(y,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,0.5x,f(x,0.5x)),(x,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,x,f(x,x)),(x,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,2x,f(x,2x)),(x,-0.5,0.5),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,-0.5x,f(x,-0.5x)),(x,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,-x,f(x,-x)),(x,-1,1),color='orange',thickness=2)
gr=gr+parametric_plot3d((x,-2x,f(x,-2*x)),(x,-0.5,0.5),color='orange',thickness=2)
show(gr)

When I changed the plot_points to 151 it worked and gave a picture. Now a few minutes later it works with 150 as well, so there is nothing wrong with the 150.

It almost feels like some sort of caching going wrong.