appendHeavy() {
    const {
      pswp
    } = this;
    const appendHeavyNearby = true; // todo
    // Avoid appending heavy elements during animations

    if (this.heavyAppended || !pswp.opener.isOpen || pswp.mainScroll.isShifted() || !this.isActive && !appendHeavyNearby) {
      return;
    }

    if (this.pswp.dispatch('appendHeavy', {
      slide: this
    }).defaultPrevented) {
      return;
    }

    this.heavyAppended = true;
    this.content.append();
    this.pswp.dispatch('appendHeavyContent', {
      slide: this
    });
  }
Append "heavy" DOM elements

This may depend on a type of slide,
but generally these are large images.