The viewer is only interactive when the node is in Viewer Active mode.
left: node with 3-letter icon. middle: node with viewer (top-left flag of node). right: node with viewer active (bottom right flag).
To get a floating resizable node viewer, right-click on the node and select View...
Node viewers can be on by default when you create new nodes. To do this, turn on the Viewer On by Default option via the checkbox in Edit > Preferences > Network.
Each operator family has its specific style of Viewer.