In "Cluster nodes into blocks" mode, it should be possible to select all nodes in a block by simply clicking in the block. The attached images illustrate the desired behavior, before and after clicking on block B2. Similarly, the current node selection should be extended with all nodes of a block when holding the Ctrl key and clicking on the block.
JDK-8289503 IdealGraphVisualizer improvements for JDK 20
JDK-8265442 IGV: select block nodes by clicking on it