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

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 { annotationManager } = instance.Core;

    annotationManager.addEventListener('annotationChanged', (annotations, action) => {
      if (action === 'add') {
        annotations.forEach((annotation) => {
          annotation.setRotationControlEnabled(false);
        });
      }
    });
  });