Customizing Block(section block) While Dragging it

If your sections always require 1 or 2 columns, why not remove the section and column blocks, instead, add new blocks for "section with 1 column" and "section with 2 columns" and save the popup?

Yeah it is possible only .But we have the requirement that we need to show single section to customer and based their pick in popup only we need to split it.

Ok. In that case, it's the canvas:drop event. Hook into that, check it's a section and open your modal. I'm not sure on how you insert the columns but hope this helps.

    editor.on('canvas:drop', (dataTransfer, model) => {
        if (model.is('section')) { // <-- might not be called 'section' 
            // Open your modal and do your thing...
        }
    });