setUpDragSession() {
    // Note: don't use any references to the editor's parent which can be null
    // if the editor belongs to a destroyed page.
    if (!this.hasSelection) {
      return;
    }
    // Avoid to have spurious text selection in the text layer when dragging.
    this.disableUserSelect(true);
    this.#draggingEditors = new Map();
    for (const editor of this.#selectedEditors) {
      this.#draggingEditors.set(editor, {
        savedX: editor.x,
        savedY: editor.y,
        savedPageIndex: editor.pageIndex,
        newX: 0,
        newY: 0,
        newPageIndex: -1,
      });
    }
  }
Set up the drag session for moving the selected editors.