Bug 1657796 - Ensure downloads requested to open with system viewer always do. r=Gijs

* Check both the useSystemViewer flag and the preferredAction for that mime-type when deciding to not open PDFs into pdf.js
* Don't set alwaysAskBeforeHandling to true when toggling off the "Always open in system viewer" context menu item.

Differential Revision: https://phabricator.services.mozilla.com/D86992

browser/components/downloads/DownloadsViewUI.jsm

@@ -856,7 +856,6 @@ DownloadsViewUI.DownloadElementShell.prototype = {
       );
       // User has selected to not open this mime-type with the system viewer
       mimeInfo.preferredAction = mimeInfo.handleInternally;
-      mimeInfo.alwaysAskBeforeHandling = true;
     }
     handlerSvc.store(mimeInfo);
     DownloadsCommon.openDownload(this.download).catch(Cu.reportError);

toolkit/components/downloads/DownloadIntegration.jsm

@@ -795,23 +795,29 @@ var DownloadIntegration = {
     }
 
     const PDF_CONTENT_TYPE = "application/pdf";
-    if (
-      aDownload.handleInternally ||
-      (!useSystemDefault && // No explicit instruction was passed to launch this download using the default system viewer.
-        mimeInfo &&
-        (mimeInfo.type == PDF_CONTENT_TYPE ||
-          fileExtension?.toLowerCase() == "pdf") &&
-        !mimeInfo.alwaysAskBeforeHandling &&
-        mimeInfo.preferredAction === Ci.nsIHandlerInfo.handleInternally &&
-        !aDownload.launchWhenSucceeded)
-    ) {
-      DownloadUIHelper.loadFileIn(file, {
-        browsingContextId: aDownload.source.browsingContextId,
-        isPrivate: aDownload.source.isPrivate,
-        openWhere,
-        userContextId: aDownload.source.userContextId,
-      });
-      return;
+
+    if (!useSystemDefault && mimeInfo) {
+      useSystemDefault = mimeInfo.preferredAction == mimeInfo.useSystemDefault;
+    }
+    if (!useSystemDefault) {
+      // No explicit instruction was passed to launch this download using the default system viewer.
+      if (
+        aDownload.handleInternally ||
+        (mimeInfo &&
+          (mimeInfo.type == PDF_CONTENT_TYPE ||
+            fileExtension?.toLowerCase() == "pdf") &&
+          !mimeInfo.alwaysAskBeforeHandling &&
+          mimeInfo.preferredAction === Ci.nsIHandlerInfo.handleInternally &&
+          !aDownload.launchWhenSucceeded)
+      ) {
+        DownloadUIHelper.loadFileIn(file, {
+          browsingContextId: aDownload.source.browsingContextId,
+          isPrivate: aDownload.source.isPrivate,
+          openWhere,
+          userContextId: aDownload.source.userContextId,
+        });
+        return;
+      }
     }
 
     // An attempt will now be made to launch the download, clear the