Preserve multiselection when shift held on right click (#117)

Fixes #112

Author: mjgallag

src/multiselect_controls.js

@@ -392,13 +392,18 @@ export class MultiselectControls {
           !(Blockly.getSelected() instanceof MultiselectDraggable)) {
         // Blockly.getSelected() is not a multiselectDraggable
         // and selected block is not in dragSelection
-        for (const draggable of this.multiDraggable.subDraggables) {
-          draggable[0].unselect();
+        if (this.dragSelection.size > 1) {
+          // Preserve multiselection when shift held on right click
+          Blockly.common.setSelected(this.multiDraggable);
+        } else {
+          for (const draggable of this.multiDraggable.subDraggables) {
+            draggable[0].unselect();
+          }
+          this.multiDraggable.clearAll_();
+          this.dragSelection.clear();
+          this.lastSelectedElement_ = Blockly.getSelected();
+          inPasteShortcut.set(this.workspace_, false);
         }
-        this.multiDraggable.clearAll_();
-        this.dragSelection.clear();
-        this.lastSelectedElement_ = Blockly.getSelected();
-        inPasteShortcut.set(this.workspace_, false);
       }
     } else {
       // In multiselect mode