Bug 1639069 - Add download context menu items to 'Use' and 'Always use' the system viewer to open the download. r=jaws

Differential Revision: https://phabricator.services.mozilla.com/D79396

browser/app/profile/firefox.js

@@ -359,6 +359,13 @@ pref("browser.download.animateNotifications", true);
 // This records whether or not the panel has been shown at least once.
 pref("browser.download.panel.shown", false);
 
+// This records whether or not to show the 'Open in system viewer' context menu item when appropriate
+pref("browser.download.openInSystemViewerContextMenuItem", true);
+
+// This records whether or not to show the 'Always open...' context menu item when appropriate
+pref("browser.download.alwaysOpenInSystemViewerContextMenuItem", true);
+
+
 // This controls whether the button is automatically shown/hidden depending
 // on whether there are downloads to show.
 pref("browser.download.autohideButton", true);

browser/components/downloads/DownloadsCommon.jsm

@@ -229,6 +229,8 @@ var PrefObserver = {
 PrefObserver.register({
   // prefName: defaultValue
   animateNotifications: true,
+  openInSystemViewerContextMenuItem: true,
+  alwaysOpenInSystemViewerContextMenuItem: true,
 });
 
 // DownloadsCommon
@@ -296,6 +298,20 @@ var DownloadsCommon = {
     return PrefObserver.animateNotifications;
   },
 
+  /**
+   * Indicates whether or not to show the 'Open in system viewer' context menu item when appropriate
+   */
+  get openInSystemViewerItemEnabled() {
+    return PrefObserver.openInSystemViewerContextMenuItem;
+  },
+
+  /**
+   * Indicates whether or not to show the 'Always open...' context menu item when appropriate
+   */
+  get alwaysOpenInSystemViewerItemEnabled() {
+    return PrefObserver.alwaysOpenInSystemViewerContextMenuItem;
+  },
+
   /**
    * Get access to one of the DownloadsData, PrivateDownloadsData, or
    * HistoryDownloadsData objects, depending on the privacy status of the
@@ -610,6 +626,9 @@ var DownloadsCommon = {
    * @param options.openWhere
    *        Optional string indicating how to handle opening a download target file URI.
    *        One of "window", "tab", "tabshifted".
+   * @param options.useSystemDefault
+   *        Optional value indicating how to handle launching this download,
+   *        this call only. Will override the associated mimeInfo.preferredAction
    * @return {Promise}
    * @resolves When the instruction to launch the file has been
    *           successfully given to the operating system or handled internally

browser/components/downloads/DownloadsSubview.jsm

@@ -297,6 +297,11 @@ class DownloadsSubview extends DownloadsViewUI.BaseView {
     while (!button._shell) {
       button = button.parentNode;
     }
+    let download = button._shell.download;
+    let { preferredAction, useSystemDefault } = DownloadsCommon.getMimeInfo(
+      download
+    );
+
     menu.setAttribute("state", button.getAttribute("state"));
     if (button.hasAttribute("exists")) {
       menu.setAttribute("exists", button.getAttribute("exists"));
@@ -307,6 +312,37 @@ class DownloadsSubview extends DownloadsViewUI.BaseView {
       "temporary-block",
       button.classList.contains("temporary-block")
     );
+    // menu items are conditionally displayed via CSS based on an is-pdf attribute
+    DownloadsCommon.log(
+      "DownloadsSubview, updateContextMenu, download is pdf? ",
+      download.target.path,
+      button.hasAttribute("is-pdf")
+    );
+    if (button.hasAttribute("is-pdf")) {
+      menu.setAttribute("is-pdf", "true");
+      let alwaysUseSystemViewerItem = menu.querySelector(
+        ".downloadAlwaysUseSystemDefaultMenuItem"
+      );
+      if (preferredAction === useSystemDefault) {
+        alwaysUseSystemViewerItem.setAttribute("checked", "true");
+      } else {
+        alwaysUseSystemViewerItem.removeAttribute("checked");
+      }
+      alwaysUseSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.alwaysOpenInSystemViewerItemEnabled
+      );
+      let useSystemViewerItem = menu.querySelector(
+        ".downloadUseSystemDefaultMenuItem"
+      );
+      useSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.openInSystemViewerItemEnabled
+      );
+    } else {
+      menu.removeAttribute("is-pdf");
+    }
+
     for (let menuitem of menu.getElementsByTagName("menuitem")) {
       let command = menuitem.getAttribute("command");
       if (!command) {

browser/components/downloads/DownloadsViewUI.jsm

@@ -24,6 +24,13 @@ XPCOMUtils.defineLazyModuleGetters(this, {
   OS: "resource://gre/modules/osfile.jsm",
 });
 
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "handlerSvc",
+  "@mozilla.org/uriloader/handler-service;1",
+  "nsIHandlerService"
+);
+
 const HTML_NS = "http://www.w3.org/1999/xhtml";
 
 var gDownloadElementButtons = {
@@ -457,9 +464,21 @@ DownloadsViewUI.DownloadElementShell.prototype = {
       // on other properties. The order in which we check the properties of the
       // Download object is the same used by stateOfDownload.
       if (this.download.succeeded) {
+        DownloadsCommon.log(
+          "_updateStateInner, target exists? ",
+          this.download.target.path,
+          this.download.target.exists
+        );
         if (this.download.target.exists) {
           // This is a completed download, and the target file still exists.
           this.element.setAttribute("exists", "true");
+
+          const isPDF = DownloadsCommon.isFileOfType(
+            this.download,
+            "application/pdf"
+          );
+          this.element.toggleAttribute("is-pdf", isPDF);
+
           let sizeWithUnits = DownloadsViewUI.getSizeWithUnits(this.download);
           if (this.isPanel) {
             // In the Downloads Panel, we show the file size after the state
@@ -728,6 +747,9 @@ DownloadsViewUI.DownloadElementShell.prototype = {
       case "cmd_delete":
         // We don't want in-progress downloads to be removed accidentally.
         return this.download.stopped;
+      case "downloadsCmd_openInSystemViewer":
+      case "downloadsCmd_alwaysOpenInSystemViewer":
+        return DownloadsCommon.isFileOfType(this.download, "application/pdf");
     }
     return DownloadsViewUI.isCommandName(aCommand) && !!this[aCommand];
   },
@@ -807,4 +829,39 @@ DownloadsViewUI.DownloadElementShell.prototype = {
   cmd_delete() {
     DownloadsCommon.deleteDownload(this.download).catch(Cu.reportError);
   },
+
+  downloadsCmd_openInSystemViewer() {
+    // For this interaction only, pass a flag to override the preferredAction for this
+    // mime-type and open using the system viewer
+    DownloadsCommon.openDownload(this.download, {
+      useSystemDefault: true,
+    }).catch(Cu.reportError);
+  },
+
+  downloadsCmd_alwaysOpenInSystemViewer() {
+    // this command toggles between setting preferredAction for this mime-type to open
+    // using the system viewer, or to open the file in browser.
+    const mimeInfo = DownloadsCommon.getMimeInfo(this.download);
+    if (mimeInfo.preferredAction !== mimeInfo.useSystemDefault) {
+      // User has selected to open this mime-type with the system viewer from now on
+      DownloadsCommon.log(
+        "downloadsCmd_alwaysOpenInSystemViewer command for download: ",
+        this.download,
+        "switching to use system default for " + mimeInfo.type
+      );
+      mimeInfo.preferredAction = mimeInfo.useSystemDefault;
+      mimeInfo.alwaysAskBeforeHandling = false;
+    } else {
+      DownloadsCommon.log(
+        "downloadsCmd_alwaysOpenInSystemViewer command for download: ",
+        this.download,
+        "currently uses system default, switching to handleInternally"
+      );
+      // 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);
+  },
 };

browser/components/downloads/content/allDownloadsView.js

@@ -705,6 +705,9 @@ DownloadsPlacesView.prototype = {
     // Set the state attribute so that only the appropriate items are displayed.
     let contextMenu = document.getElementById("downloadsContextMenu");
     let download = element._shell.download;
+    let { preferredAction, useSystemDefault } = DownloadsCommon.getMimeInfo(
+      download
+    );
     contextMenu.setAttribute(
       "state",
       DownloadsCommon.stateOfDownload(download)
@@ -712,6 +715,31 @@ DownloadsPlacesView.prototype = {
     contextMenu.setAttribute("exists", "true");
     contextMenu.classList.toggle("temporary-block", !!download.hasBlockedData);
 
+    if (element.hasAttribute("is-pdf")) {
+      contextMenu.setAttribute("is-pdf", "true");
+      let alwaysUseSystemViewerItem = contextMenu.querySelector(
+        ".downloadAlwaysUseSystemDefaultMenuItem"
+      );
+      if (preferredAction === useSystemDefault) {
+        alwaysUseSystemViewerItem.setAttribute("checked", "true");
+      } else {
+        alwaysUseSystemViewerItem.removeAttribute("checked");
+      }
+      alwaysUseSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.alwaysOpenInSystemViewerItemEnabled
+      );
+      let useSystemViewerItem = contextMenu.querySelector(
+        ".downloadUseSystemDefaultMenuItem"
+      );
+      useSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.openInSystemViewerItemEnabled
+      );
+    } else {
+      contextMenu.removeAttribute("is-pdf");
+    }
+
     if (!download.stopped) {
       // The hasPartialData property of a download may change at any time after
       // it has started, so ensure we update the related command now.

browser/components/downloads/content/downloads.css

@@ -95,8 +95,14 @@
 .download-state[state="1"]:not([exists])
                                            .downloadCommandsSeparator,
 .download-state[state="8"]:not(.temporary-block)
-                                           .downloadCommandsSeparator
+                                           .downloadCommandsSeparator,
 
+/* the system-viewer context menu items are only shown for certain mime-types
+   and can be individually enabled via prefs */
+.download-state:not([is-pdf])              .downloadUseSystemDefaultMenuItem,
+.download-state                            .downloadUseSystemDefaultMenuItem:not([enabled]),
+.download-state                            .downloadAlwaysUseSystemDefaultMenuItem:not([enabled]),
+.download-state:not([is-pdf])              .downloadAlwaysUseSystemDefaultMenuItem
 {
   display: none;
 }

browser/components/downloads/content/downloads.js

@@ -937,6 +937,11 @@ var DownloadsView = {
 
     DownloadsViewController.updateCommands();
 
+    let download = element._shell.download;
+    let { preferredAction, useSystemDefault } = DownloadsCommon.getMimeInfo(
+      download
+    );
+
     // Set the state attribute so that only the appropriate items are displayed.
     let contextMenu = document.getElementById("downloadsContextMenu");
     contextMenu.setAttribute("state", element.getAttribute("state"));
@@ -949,6 +954,30 @@ var DownloadsView = {
       "temporary-block",
       element.classList.contains("temporary-block")
     );
+    if (element.hasAttribute("is-pdf")) {
+      contextMenu.setAttribute("is-pdf", "true");
+      let alwaysUseSystemViewerItem = contextMenu.querySelector(
+        ".downloadAlwaysUseSystemDefaultMenuItem"
+      );
+      if (preferredAction === useSystemDefault) {
+        alwaysUseSystemViewerItem.setAttribute("checked", "true");
+      } else {
+        alwaysUseSystemViewerItem.removeAttribute("checked");
+      }
+      alwaysUseSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.alwaysOpenInSystemViewerItemEnabled
+      );
+      let useSystemViewerItem = contextMenu.querySelector(
+        ".downloadUseSystemDefaultMenuItem"
+      );
+      useSystemViewerItem.toggleAttribute(
+        "enabled",
+        DownloadsCommon.openInSystemViewerItemEnabled
+      );
+    } else {
+      contextMenu.removeAttribute("is-pdf");
+    }
   },
 
   onDownloadDragStart(aEvent) {
@@ -1093,6 +1122,22 @@ class DownloadsViewItem extends DownloadsViewUI.DownloadElementShell {
     DownloadsPanel.hidePanel();
   }
 
+  downloadsCmd_openInSystemViewer() {
+    super.downloadsCmd_openInSystemViewer();
+
+    // We explicitly close the panel here to give the user the feedback that
+    // their click has been received, and we're handling the action.
+    DownloadsPanel.hidePanel();
+  }
+
+  downloadsCmd_alwaysOpenInSystemViewer() {
+    super.downloadsCmd_alwaysOpenInSystemViewer();
+
+    // We explicitly close the panel here to give the user the feedback that
+    // their click has been received, and we're handling the action.
+    DownloadsPanel.hidePanel();
+  }
+
   downloadsCmd_show() {
     let file = new FileUtils.File(this.download.target.path);
     DownloadsCommon.showDownloadedFile(file);

browser/components/downloads/content/downloadsCommands.inc.xhtml

@@ -22,4 +22,6 @@
   <command id="downloadsCmd_retry"/>
   <command id="downloadsCmd_openReferrer"/>
   <command id="downloadsCmd_clearDownloads"/>
+  <command id="downloadsCmd_openInSystemViewer"/>
+  <command id="downloadsCmd_alwaysOpenInSystemViewer"/>
 </commandset>

browser/components/downloads/content/downloadsContextMenu.inc.xhtml

@@ -17,6 +17,13 @@
   <menuitem command="downloadsCmd_unblock"
             class="downloadUnblockMenuItem"
             data-l10n-id="downloads-cmd-unblock"/>
+  <menuitem command="downloadsCmd_openInSystemViewer"
+            class="downloadUseSystemDefaultMenuItem"
+            data-l10n-id="downloads-cmd-use-system-default"/>
+  <menuitem command="downloadsCmd_alwaysOpenInSystemViewer"
+            type="checkbox"
+            class="downloadAlwaysUseSystemDefaultMenuItem"
+            data-l10n-id="downloads-cmd-always-use-system-default"/>
   <menuitem command="downloadsCmd_show"
             class="downloadShowMenuItem"
 #ifdef XP_MACOSX

browser/components/downloads/content/downloadsPanel.inc.xhtml

@@ -38,6 +38,10 @@
            oncommand="goDoCommand('downloadsCmd_copyLocation')"/>
   <command id="downloadsCmd_clearList"
            oncommand="goDoCommand('downloadsCmd_clearList')"/>
+  <command id="downloadsCmd_openInSystemViewer"
+           oncommand="goDoCommand('downloadsCmd_openInSystemViewer')"/>
+  <command id="downloadsCmd_alwaysOpenInSystemViewer"
+           oncommand="goDoCommand('downloadsCmd_alwaysOpenInSystemViewer')"/>
 </commandset>
 
 <!-- The panel has level="top" to ensure that it is never hidden by the
@@ -75,6 +79,13 @@
     <menuitem command="downloadsCmd_unblock"
               class="downloadUnblockMenuItem"
               data-l10n-id="downloads-cmd-unblock"/>
+    <menuitem command="downloadsCmd_openInSystemViewer"
+              class="downloadUseSystemDefaultMenuItem"
+              data-l10n-id="downloads-cmd-use-system-default"/>
+    <menuitem command="downloadsCmd_alwaysOpenInSystemViewer"
+              type="checkbox"
+              class="downloadAlwaysUseSystemDefaultMenuItem"
+              data-l10n-id="downloads-cmd-always-use-system-default"/>
     <menuitem command="downloadsCmd_show"
               class="downloadShowMenuItem"
   #ifdef XP_MACOSX

browser/components/downloads/test/browser/browser_pdfjs_preview.js

@@ -29,6 +29,26 @@ const TestCases = [
       tabSelected: true,
     },
   },
+  {
+    name: "Download panel, system viewer menu items prefd off",
+    whichUI: "downloadPanel",
+    itemSelector: "#downloadsListBox richlistitem .downloadMainArea",
+    async userEvents(itemTarget, win) {
+      EventUtils.synthesizeMouseAtCenter(itemTarget, {}, win);
+    },
+    prefs: [
+      ["browser.download.openInSystemViewerContextMenuItem", false],
+      ["browser.download.alwaysOpenInSystemViewerContextMenuItem", false],
+    ],
+    expected: {
+      downloadCount: 1,
+      newWindow: false,
+      opensTab: true,
+      tabSelected: true,
+      useSystemMenuItemDisabled: true,
+      alwaysMenuItemDisabled: true,
+    },
+  },
   {
     name: "Download panel, open from keyboard",
     whichUI: "downloadPanel",
@@ -109,6 +129,26 @@ const TestCases = [
       tabSelected: true,
     },
   },
+  {
+    name: "Library all downloads dialog, system viewer menu items prefd off",
+    whichUI: "allDownloads",
+    async userEvents(itemTarget, win) {
+      // double click
+      await triggerDblclickOn(itemTarget, {}, win);
+    },
+    prefs: [
+      ["browser.download.openInSystemViewerContextMenuItem", false],
+      ["browser.download.alwaysOpenInSystemViewerContextMenuItem", false],
+    ],
+    expected: {
+      downloadCount: 1,
+      newWindow: false,
+      opensTab: true,
+      tabSelected: true,
+      useSystemMenuItemDisabled: true,
+      alwaysMenuItemDisabled: true,
+    },
+  },
   {
     name: "Library all downloads dialog, open from keyboard",
     whichUI: "allDownloads",
@@ -189,6 +229,28 @@ const TestCases = [
       tabSelected: true,
     },
   },
+  {
+    name: "about:downloads, system viewer menu items prefd off",
+    whichUI: "aboutDownloads",
+    itemSelector: "#downloadsRichListBox richlistitem .downloadContainer",
+    async userEvents(itemSelector, win) {
+      let browser = win.gBrowser.selectedBrowser;
+      is(browser.currentURI.spec, "about:downloads");
+      await contentTriggerDblclickOn(itemSelector, {}, browser);
+    },
+    prefs: [
+      ["browser.download.openInSystemViewerContextMenuItem", false],
+      ["browser.download.alwaysOpenInSystemViewerContextMenuItem", false],
+    ],
+    expected: {
+      downloadCount: 1,
+      newWindow: false,
+      opensTab: true,
+      tabSelected: true,
+      useSystemMenuItemDisabled: true,
+      alwaysMenuItemDisabled: true,
+    },
+  },
   {
     name: "about:downloads, open in new window",
     whichUI: "aboutDownloads",
@@ -306,6 +368,66 @@ function contentTriggerDblclickOn(selector, eventModifiers = {}, browser) {
   );
 }
 
+async function openContextMenu(itemElement, win = window) {
+  let popupShownPromise = BrowserTestUtils.waitForEvent(
+    itemElement.ownerDocument,
+    "popupshown"
+  );
+  EventUtils.synthesizeMouseAtCenter(
+    itemElement,
+    {
+      type: "contextmenu",
+      button: 2,
+    },
+    win
+  );
+  let { target } = await popupShownPromise;
+  return target;
+}
+
+async function verifyContextMenu(contextMenu, expected = {}) {
+  info("verifyContextMenu with expected: " + JSON.stringify(expected, null, 2));
+  let alwaysMenuItem = contextMenu.querySelector(
+    ".downloadAlwaysUseSystemDefaultMenuItem"
+  );
+  let useSystemMenuItem = contextMenu.querySelector(
+    ".downloadUseSystemDefaultMenuItem"
+  );
+  await TestUtils.waitForCondition(
+    () => BrowserTestUtils.is_visible(contextMenu),
+    "The context menu is visible"
+  );
+  await TestUtils.waitForTick();
+
+  is(
+    BrowserTestUtils.is_hidden(useSystemMenuItem),
+    expected.useSystemMenuItemDisabled,
+    `The 'Use system viewer' menu item was ${
+      expected.useSystemMenuItemDisabled ? "hidden" : "visible"
+    }`
+  );
+  is(
+    BrowserTestUtils.is_hidden(alwaysMenuItem),
+    expected.alwaysMenuItemDisabled,
+    `The 'Use system viewer' menu item was ${
+      expected.alwaysMenuItemDisabled ? "hidden" : "visible"
+    }`
+  );
+
+  if (!expected.useSystemMenuItemDisabled && expected.alwaysChecked) {
+    is(
+      alwaysMenuItem.getAttribute("checked"),
+      "true",
+      "The 'Always...' menu item is checked"
+    );
+  } else if (!expected.useSystemMenuItemDisabled) {
+    ok(
+      !alwaysMenuItem.hasAttribute("checked"),
+      "The 'Always...' menu item not checked"
+    );
+  }
+}
+
 async function createDownloadedFile(pathname, contents) {
   let encoder = new TextEncoder();
   let file = new FileUtils.File(pathname);
@@ -352,7 +474,7 @@ async function addPDFDownload(itemData) {
   return download;
 }
 
-async function testSetup(testData = {}) {
+async function testSetup() {
   // remove download files, empty out collections
   let downloadList = await Downloads.getList(Downloads.ALL);
   let downloadCount = (await downloadList.getAll()).length;
@@ -379,6 +501,7 @@ async function testOpenPDFPreview({
   whichUI,
   itemSelector,
   expected,
+  prefs = [],
   userEvents,
   isPrivate,
 }) {
@@ -386,6 +509,11 @@ async function testOpenPDFPreview({
   // Wait for focus first
   await promiseFocus();
   await testSetup();
+  if (prefs.length) {
+    await SpecialPowers.pushPrefEnv({
+      set: prefs,
+    });
+  }
 
   // Populate downloads database with the data required by this test.
   info("Adding download objects");
@@ -454,12 +582,16 @@ async function testOpenPDFPreview({
     }
 
     let itemTarget;
+    let contextMenu;
+
     switch (whichUI) {
       case "downloadPanel":
         info("Opening download panel");
         await openDownloadPanel(expected.downloadCount);
         info("/Opening download panel");
         itemTarget = document.querySelector(itemSelector);
+        contextMenu = uiWindow.document.querySelector("#downloadsContextMenu");
+
         break;
       case "allDownloads":
         // we'll be interacting with the library dialog
@@ -467,11 +599,17 @@ async function testOpenPDFPreview({
 
         let listbox = uiWindow.document.getElementById("downloadsRichListBox");
         ok(listbox, "download list box present");
-        // wait for the expected number of items in the view
+        // wait for the expected number of items in the view,
-        await TestUtils.waitForCondition(
+        // and for the first item to be visible && clickable
-          () => listbox.itemChildren.length == expected.downloadCount
+        await TestUtils.waitForCondition(() => {
-        );
+          return (
+            listbox.itemChildren.length == expected.downloadCount &&
+            BrowserTestUtils.is_visible(listbox.itemChildren[0])
+          );
+        });
         itemTarget = listbox.itemChildren[0];
+        contextMenu = uiWindow.document.querySelector("#downloadsContextMenu");
+
         break;
       case "aboutDownloads":
         info("Preparing about:downloads browser window");
@@ -537,6 +675,21 @@ async function testOpenPDFPreview({
         break;
     }
 
+    if (contextMenu) {
+      info("trigger the contextmenu");
+      await openContextMenu(itemTarget || itemSelector, uiWindow);
+      info("context menu should be open, verify its menu items");
+      let expectedValues = {
+        useSystemMenuItemDisabled: false,
+        alwaysMenuItemDisabled: false,
+        ...expected,
+      };
+      await verifyContextMenu(contextMenu, expectedValues);
+      contextMenu.hidePopup();
+    } else {
+      todo(contextMenu, "No context menu checks for test: " + name);
+    }
+
     info("Executing user events");
     await userEvents(itemTarget || itemSelector, uiWindow);
 
@@ -594,6 +747,9 @@ async function testOpenPDFPreview({
     await lastPBContextExitedPromise;
   });
   await downloadList.removeFinished();
+  if (prefs.length) {
+    await SpecialPowers.popPrefEnv();
+  }
 }
 
 // register the tests

toolkit/components/downloads/DownloadCore.jsm

@@ -726,6 +726,9 @@ Download.prototype = {
    * @param options.openWhere  Optional string indicating how to open when handling
    *                           download by opening the target file URI.
    *                           One of "window", "tab", "tabshifted"
+   * @param options.useSystemDefault
+   *                           Optional value indicating how to handle launching this download,
+   *                           this time only. Will override the associated mimeInfo.preferredAction
    * @return {Promise}
    * @resolves When the instruction to launch the file has been
    *           successfully given to the operating system. Note that

toolkit/components/downloads/DownloadIntegration.jsm

@@ -712,6 +712,9 @@ var DownloadIntegration = {
    * @param options.openWhere     Optional string indicating how to open when handling
    *                              download by opening the target file URI.
    *                              One of "window", "tab", "tabshifted"
+   * @param options.useSystemDefault
+   *                              Optional value indicating how to handle launching this download,
+   *                              this time only. Will override the associated mimeInfo.preferredAction
    *
    * @return {Promise}
    * @resolves When the instruction to launch the file has been
@@ -721,7 +724,7 @@ var DownloadIntegration = {
    * @rejects  JavaScript exception if there was an error trying to launch
    *           the file.
    */
-  async launchDownload(aDownload, { openWhere }) {
+  async launchDownload(aDownload, { openWhere, useSystemDefault = null }) {
     let file = new FileUtils.File(aDownload.target.path);
 
     // In case of a double extension, like ".tar.gz", we only
@@ -794,7 +797,8 @@ var DownloadIntegration = {
     const PDF_CONTENT_TYPE = "application/pdf";
     if (
       aDownload.handleInternally ||
-      (mimeInfo &&
+      (!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 &&