No more previews in compositor In Node Editor (Compositing) there are no previews in the "Viewer" and "Composite" nodes. The Backdrop works and the "Render Layers" nodes also show their preview.

I found this question but this doesn't help.
I can do F12 as much as I like. There are no previews in compositor.

So it looks currently:



I want it to look that way (screenshot from a YouTube video):



Is there an option I could have disabled accidentally?

(I am using Blender Render engine, not Cycles.)
In the Compositor


Select Node First. Toggle Node Preview.







Make sure you click on a Viewer.  It should have a highlight.
Make sure Backdrop (Show Backdrop) is enabled.