From c6ed7347eefc393cfebc51cbf422571687addd2c Mon Sep 17 00:00:00 2001
From: Jonathan Kew <jkew@mozilla.com>
Date: Wed, 2 Jul 2025 12:25:32 +0000
Subject: [PATCH] Bug 1959052 - Ensure JS execution is blocked during
 window.print() when we're using the system UI (rather than printing via the
 Firefox preview panel).  a=pascalc

The patch in bug 1898184 made reasonable sense, inasmuch as when the
print.prefer_system_dialog option is set, we don't show our window-modal
preview panel.

However, as noted in comment 6 here, we do still need to block JS while
window.print() is presenting the system dialog, otherwise the document
may be changed or even closed by script behind the dialog's back. Prior
to bug 1898184, the IsPreview::Yes flag caused us to block, but the issue
was that without the preview panel taking care of cleaning up the cloned
document, the block got cleared.

It looks like we can solve this by setting the closeWindowAfterPrint
flag in this case. In local testing, this seems to work for the various
examples here, and does not regress bug 1898184.

Original Revision: https://phabricator.services.mozilla.com/D253655

Differential Revision: https://phabricator.services.mozilla.com/D253944
---
 dom/base/nsGlobalWindowOuter.cpp         | 15 ++++++++++-----
 modules/libpref/init/StaticPrefList.yaml |  8 ++++++++
 modules/libpref/init/all.js              |  5 -----
 3 files changed, 18 insertions(+), 10 deletions(-)

diff --git a/dom/base/nsGlobalWindowOuter.cpp b/dom/base/nsGlobalWindowOuter.cpp
index 42ee50b53a66..b5cf9524c493 100644
--- a/dom/base/nsGlobalWindowOuter.cpp
+++ b/dom/base/nsGlobalWindowOuter.cpp
@@ -4937,9 +4937,8 @@ void nsGlobalWindowOuter::PrintOuter(ErrorResult& aError) {
     }
   });
 
-  const bool forPreview =
-      !StaticPrefs::print_always_print_silent() &&
-      !Preferences::GetBool("print.prefer_system_dialog", false);
+  const bool forPreview = !StaticPrefs::print_always_print_silent() &&
+                          !StaticPrefs::print_prefer_system_dialog();
   Print(nullptr, nullptr, nullptr, nullptr, IsPreview(forPreview),
         IsForWindowDotPrint::Yes, nullptr, nullptr, aError);
 #endif
@@ -5136,8 +5135,11 @@ Nullable<WindowProxyHolder> nsGlobalWindowOuter::Print(
     // The exception is if we're using the passed-in aCachedBrowsingContext, in
     // which case this is the second print with this static document clone that
     // we created the first time through, and we are responsible for cleaning it
-    // up.
-    closeWindowAfterPrint = usingCachedBrowsingContext;
+    // up. There's also an exception if we're directly using the system print
+    // dialog rather than our preview panel, because in this case the preview
+    // will not take care of cleaning up the cloned doc.
+    closeWindowAfterPrint =
+        usingCachedBrowsingContext || StaticPrefs::print_prefer_system_dialog();
   } else {
     // In this case the document was not a static clone, so we made a static
     // clone for printing purposes and must clean it up after the print is done.
@@ -5179,6 +5181,9 @@ Nullable<WindowProxyHolder> nsGlobalWindowOuter::Print(
     if (aIsPreview == IsPreview::Yes) {
       return !hasPrintCallbacks;
     }
+    if (StaticPrefs::print_prefer_system_dialog()) {
+      return true;
+    }
     return StaticPrefs::dom_window_print_fuzzing_block_while_printing();
   }();
 
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index acadd3a2cca1..283167eb4d32 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -15589,6 +15589,14 @@
   value: false
   mirror: always
 
+# Whether we directly use the system print dialog to collect the user's print
+# settings rather than using the tab-modal print preview dialog.
+# Note: `print.always_print_silent` overrides this.
+- name: print.prefer_system_dialog
+  type: RelaxedAtomicBool
+  value: false
+  mirror: always
+
 # Whether we attempt to generate links in Save As PDF output.
 - name: print.save_as_pdf.links.enabled
   type: RelaxedAtomicBool
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 95bcd7179309..c175e7707cf3 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -725,11 +725,6 @@ pref("browser.fixup.fallback-to-https", true);
 // used in this case.  See nsPrintSettingsService::InitPrintSettingsFromPrefs
 // for the restrictions on which prefs can act as defaults.
 
-// Whether we directly use the system print dialog to collect the user's print
-// settings rather than using the tab-modal print preview dialog.
-// Note: `print.always_print_silent` overrides this.
-pref("print.prefer_system_dialog", false);
-
 // Print/Preview Shrink-To-Fit won't shrink below 20% for text-ish documents.
 pref("print.shrink-to-fit.scale-limit-percent", 20);