Line, Polyline, Polygon or Freehand annotations show a rotation control when selected. This control handle lets the user rotate the annotation with a full range (360 degrees).

linkDisabling freeform rotation

By default, freeform rotation is enabled for all Line, Polyline, Polygon or Freehand annotations. In order to disable it, the following can be done:

Webviewer(...)
  .then(instance => {
    const { annotManager } = instance;
    
    annotManager.on('annotationChanged', (annotations, action) => {
      if (action === 'add') {
        annotations.forEach((annotation) => {
          annotation.setRotationControlEnabled(false);
        });
      }
    });
  });