After having discussed yesterday with Yuriy, I gave a try at having multiple selection in Roassal. If you update Roassal, you can try:

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b := RTMondrian new.
b nodes: Collection withAllSubclasses.
b layout flow.
b normalizer 
normalizeSize: #numberOfMethods.

b build.

"Adding a label"
labelShape := RTLabel new text: 'Nothing selected'.
label := labelShape element.
b view add: label.
TRConstraint move: label below: b view elements.

"Changing the label according to the selection"
b view elements @ (RTSelectable new selectionCallback: [ :es | label trachelShape text: es size asString, ' selected elements']).

b view

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Try clicking on the square and the white background.

Wouldn’t it be nice to connect this with GTInspector?

Cheers,

Alexandre