Viewer
You can select single/multiple window mode of the viewer (View->Same Window).
If multiple windows are already on screen, new windows will appear in the current opened window.
In auto decompile mode (View->Auto Decompile) selection of the method node
causes method decompilation (Method->C#). Decompiled code appears in the same window,
irrelative of Same Window switch status.
There is the list of the nontrivial Viewer Keys.