In the following spots a "MouseUp" eventName is being used, which does look inconsistent considering that the "Mouse Down"/"Mouse Enter"/"Mouse Exit" eventNames all have a space in them.
Is this inconsistency actually expected, or is it a simple typo? If the former, then a comment explaining *why* would be appropriate; otherwise we should just fix this :-)

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L806

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L945

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L1034

 - https://github.com/mozilla/pdf.js/blob/25b84ce84c95ba06d1ee57e578672048269c39c1/src/display/annotation_layer.js#L1168

/cc @calixteman 
const BACKGROUND_ENLIGHT = 0.7;
    const wrapper = document.createElement("div");
    wrapper.className = "popupWrapper";
    
    
    
    
    this.hideElement = this.hideWrapper ? wrapper : this.container;
    this.hideElement.setAttribute("hidden", true);
    const popup = document.createElement("div");
    popup.className = "popup";
    const color = this.color;
    if (color) {
      
      const r = BACKGROUND_ENLIGHT * (255 - color[0]) + color[0];
      const g = BACKGROUND_ENLIGHT * (255 - color[1]) + color[1];
      const b = BACKGROUND_ENLIGHT * (255 - color[2]) + color[2];
      popup.style.backgroundColor = Util.makeHexColor(r | 0, g | 0, b | 0);
    }
    const title = document.createElement("h1");
    title.textContent = this.title;
    popup.appendChild(title);
    
    
    
    const dateObject = PDFDateString.toDateObject(this.modificationDate);
    if (dateObject) {
      const modificationDate = document.createElement("span");
      modificationDate.textContent = "{{date}}, {{time}}";
      modificationDate.dataset.l10nId = "annotation_date_string";