Bug 1675483, only hide sheet count when the preview is loading to avoid flicker r=sfoster

Differential Revision: https://phabricator.services.mozilla.com/D96088

toolkit/components/printing/content/print.css

@@ -80,7 +80,7 @@ body[loading] #print {
   margin: 0;
 }
 
-body[rendering] #sheet-count,
+#sheet-count[loading],
 body[invalid] #sheet-count {
   visibility: hidden;
 }

toolkit/components/printing/content/print.js

@@ -2206,6 +2206,12 @@ class PageCount extends PrintUIControlMixin(HTMLElement) {
     document.l10n.setAttributes(this, "printui-sheets-count", {
       sheetCount: this.sheetCount * this.numCopies,
     });
+
+    // The loading attribute must be removed on first render
+    if (this.hasAttribute("loading")) {
+      this.removeAttribute("loading");
+    }
+
     if (this.id) {
       // We're showing the sheet count, so let it describe the dialog.
       document.body.setAttribute("aria-describedby", this.id);