I set my nodes size to about 200 by 60, as I want to show some content in them.

I use SpringLayout Arrange() and then Diagram ResizeToFitItems() for layout but I can't make it to prevent nodes from overlapping.

Is there a way to make sure that nodes do not overlap after  SpringLayout Arrange() is called?

sl.RepulsionFactor is 0 and multiplying it by any factor is still 0, so nothing changes.

I tried to set RepulsionFactor to numbers like +-100, +-50, +-200, +-10, +-1 etc.
The node overlap is always much worse than with default value RepulsionFactor=0.

Setting EnableClusters = true improves layout some,
but I still can't get rid of node overlap...

Yes AnnealLayout is much nicer, but it's VERY slow.

I make diagrams with 25-50 nodes and it can go for 10 sec or more on high-end machine.

Also it occasionally put all the nodes in one narrow horizontal strip at the top  of the diagram right on top of each other - I suspect there is some bug there.

I designed a compromise by using both layouts with AnnealLayout  running with very few iterations to correct shortcomings of SpringLayout:

           var sl = new SpringLayout
                        {
                            NodeDistance = 200,
                            MinimizeCrossings = true,
                            RepulsionFactor = 0,
                            IterationCount = 500,
                            EnableClusters = true,
                            Anchoring = Anchoring.Reassign
                        };
           sl.Arrange(diagram);

           var al = new AnnealLayout
                        {
                            Anchoring = Anchoring.Reassign,
                            BoundaryFactor = 100000,
                            CrossingLinksCost = 300000,
                            DistributionFactor = 100000,
                            KeepGroupLayout = false,
                            IterationsPerStage = 5,
                            LinkLengthFactor = 0.10, // larger makes shorter links
                            NodeLinkCrossingCost = 250000,
                            NodeLinkDistFactor = 500000,
                            Randomize = false,
                            Stages = 10
                        };

           al.Arrange(diagram);
           diagram.ResizeToFitItems(50);

I do add a few nodes by clicking my button on my control node, which connects one or more new nodes with current node.

Is there a way to keep layout as close as possible to the layout before adding a node or two?
All nodes often end up in completely different places which is hard for users to follow.

We have ported a SpringLayout feature from the WinForms version that could help for that:
https://mindfusion.eu/_beta/diaglite14_spring.zip

Now when you need to add only a few nodes to the layout, you can set node.LayoutTraits[SpringLayoutTraits.Frozen] = true for the already arranged nodes. The layout method keeps such nodes where they are, but still considers the forces they exert in the simulated system, so the newly added nodes should be placed at a suitable position in the layout. Then you might run just a few additional iterations with the Frozen flag removed to adjust the positions of nodes adjacent to the newly added ones.

I hope that helps,
Stoyan

Do you mean the images disappear from user-controls hosted in ControlNodes?