Disable selection clear when clicked on empty canvas

GoJS
1

Currently, when a node is selected in the graph, clicking the empty space in the canvas clears the selection.

I see that Diagram has a property called maxSelectionCount for changing the maximum number of nodes that can be selected. I was thinking if there is some way to manually implement functionality that would be similar to:

diagram.minSelectionCount = 1
to make sure clicking on the empty canvas does not deselect the only selected node.

2

Override ClickSelectingTool.standardMouseSelect to do what you want.

Here is its definition in the base Tool class, Tool | GoJS API :

  public standardMouseSelect(): void {
    const diagram = this.diagram;
    if (!diagram.allowSelect) return;
    const e = diagram.lastInput;
    const curobj = diagram.findPartAt(e.documentPoint, false);  // to select containing Group if Part.canSelect() is false
    if (curobj !== null) {
      if (e.meta || e.control) {  // toggle the part's selection
        diagram.raiseDiagramEvent('ChangingSelection', diagram.selection);
        let part = curobj;
        while (part !== null && !part.canSelect()) part = part.containingGroup;
        if (part !== null) part.isSelected = !part.isSelected;
        diagram.raiseDiagramEvent('ChangedSelection', diagram.selection);
      } else if (e.shift) {  // add the part to the selection
        if (!curobj.isSelected) {
          diagram.raiseDiagramEvent('ChangingSelection', diagram.selection);
          let part = curobj;
          while (part !== null && !part.canSelect()) part = part.containingGroup;
          if (part !== null) part.isSelected = true;
          diagram.raiseDiagramEvent('ChangedSelection', diagram.selection);
        }
      } else {
        if (!curobj.isSelected) {
          let part = curobj;
          while (part !== null && !part.canSelect()) part = part.containingGroup;
          if (part !== null) diagram.select(part);  // also raises ChangingSelection/Finished
        }
      }
    } else if (e.left && !(e.meta || e.control) && !e.shift) {
      // left click on background with no modifier: clear selection
      diagram.clearSelection();  // also raises ChangingSelection/Finished
    }
  }

Note the call to Diagram.clearSelection, which you want to avoid here.