diff --git a/patches/series b/patches/series
index ebc739d9..698c4028 100644
--- a/patches/series
+++ b/patches/series
@@ -2,6 +2,7 @@ upstream-fixes/missing-dependencies.patch
 upstream-fixes/fix-crash-without-enterprise-cloud-content-analysis.patch
 upstream-fixes/fix-python-codecs-deprecation.patch
 upstream-fixes/fix-macos-widget-rect.patch
+upstream-fixes/fix-macos-fullscreen-ui-breakage.patch
 
 contrib/inox-patchset/fix-building-without-safebrowsing.patch
 contrib/inox-patchset/disable-autofill-download-manager.patch
diff --git a/patches/upstream-fixes/fix-macos-fullscreen-ui-breakage.patch b/patches/upstream-fixes/fix-macos-fullscreen-ui-breakage.patch
new file mode 100644
index 00000000..0be79658
--- /dev/null
+++ b/patches/upstream-fixes/fix-macos-fullscreen-ui-breakage.patch
@@ -0,0 +1,130 @@
+From 1e69bde8508191f45357b8c027bcd63a58d9ea8d Mon Sep 17 00:00:00 2001
+From: Dana Fried <dfried@chromium.org>
+Date: Fri, 31 Oct 2025 11:49:22 -0700
+Subject: [PATCH] [Browser Frame] Abandon reading Mac caption button positions
+
+These were not reliable during fullscreen transitions in a way that was
+not possible to accurately filter out. This now uses the old fallback
+hard-coded constants that were being used in all PWA windows anyway.
+
+Include-Ci-Only-Tests: chromium.mac:mac15-x64-rel-tests|browser_tests
+Bug: 453717426
+Change-Id: Id102a321834c3dac0f56edcacda933c88db0b82a
+Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7107360
+Reviewed-by: Darryl James <dljames@chromium.org>
+Commit-Queue: Dana Fried <dfried@chromium.org>
+Cr-Commit-Position: refs/heads/main@{#1538756}
+---
+
+--- a/chrome/browser/ui/views/frame/browser_frame_view_mac.h
++++ b/chrome/browser/ui/views/frame/browser_frame_view_mac.h
+@@ -90,9 +90,6 @@ class BrowserFrameViewMac : public Brows
+   FRIEND_TEST_ALL_PREFIXES(BrowserFrameViewMacTest,
+                            GetCaptionButtonPlaceholderBounds);
+ 
+-  // Fetches the caption button bounds from the buttons themselves.
+-  BoundsAndMargins GetCaptionButtonBoundsNative() const;
+-
+   // Creates an inset from the caption button size which controls for which edge
+   // the captions buttons exists on. Used to position elements like the tabstrip
+   // that are adjacent to the caption buttons.
+--- a/chrome/browser/ui/views/frame/browser_frame_view_mac.mm
++++ b/chrome/browser/ui/views/frame/browser_frame_view_mac.mm
+@@ -458,96 +458,9 @@ void BrowserFrameViewMac::PaintChildren(
+ }
+ 
+ BrowserFrameViewMac::BoundsAndMargins
+-BrowserFrameViewMac::GetCaptionButtonBoundsNative() const {
+-  BoundsAndMargins result;
+-
+-  // Verify that this is not an out-of-process window.
+-  const auto native_window = GetWidget()->GetNativeWindow();
+-  if (auto* const host =
+-          views::NativeWidgetMacNSWindowHost::GetFromNativeWindow(
+-              native_window);
+-      host && host->application_host()) {
+-    return result;
+-  }
+-
+-  // Verify that there is a valid NSWindow.
+-  NSWindow* ns_window = native_window.GetNativeNSWindow();
+-  if (!ns_window) {
+-    return result;
+-  }
+-
+-  // Build a list of caption button bounds.
+-  std::vector<gfx::RectF> button_rects;
+-
+-  // Also track some other data that will be useful for calculating the visual
+-  // margins of the buttons against the window border.
+-  float min_x = width();
+-  float max_x = 0;
+-  float min_y = height();
+-
+-  // Chrome coordinates are reversed in RTL but not in the Mac API, so we might
+-  // have to flip them.
+-  const bool is_rtl = base::i18n::IsRTL();
+-
+-  // Build the list. If any of the buttons are not present or are zero size,
+-  // abort (this will fall back to previous hard-coded values).
+-  for (NSButton* button :
+-       {[ns_window standardWindowButton:NSWindowCloseButton],
+-        [ns_window standardWindowButton:NSWindowMiniaturizeButton],
+-        [ns_window standardWindowButton:NSWindowZoomButton]}) {
+-    if (!button) {
+-      return result;
+-    }
+-    NSRect ns_rect = [button convertRect:[button bounds] toView:nil];
+-
+-    // When converting from Mac to Chrome coordinates:
+-    //  - Y axis is inverted (Mac coordinates start at bottom-left).
+-    //  - X axis is inverted in RTL mode only (Mac coordinates are invariant
+-    //    while Chrome reverses them).
+-    button_rects.emplace_back(
+-        is_rtl ? width() - (ns_rect.origin.x + ns_rect.size.width)
+-               : ns_rect.origin.x,
+-        height() - (ns_rect.origin.y + ns_rect.size.height), ns_rect.size.width,
+-        ns_rect.size.height);
+-    const auto& rect = button_rects.back();
+-    if (rect.IsEmpty()) {
+-      return result;
+-    }
+-    min_x = std::min(min_x, rect.x());
+-    max_x = std::max(max_x, rect.right());
+-    min_y = std::min(min_y, rect.y());
+-  }
+-
+-  // Calculate the margins that the buttons are using.
+-  const float block_margin = min_y;
+-  const float inline_margin =
+-      CaptionButtonsOnLeadingEdge() ? min_x : width() - max_x;
+-
+-  // Accumulate the button bounds.
+-  for (const auto& rect : button_rects) {
+-    result.bounds.Union(rect);
+-  }
+-
+-  // Apply the margins on the exterior of the region, so that the padding around
+-  // the buttons appears visually symmetrical.
+-  result.margins = gfx::OutsetsF::VH(block_margin, inline_margin);
+-
+-  return result;
+-}
+-
+-BrowserFrameViewMac::BoundsAndMargins
+ BrowserFrameViewMac::GetCaptionButtonBounds() const {
+-  BoundsAndMargins result = GetCaptionButtonBoundsNative();
+-  if (!result.bounds.IsEmpty()) {
+-    return result;
+-  }
++  BoundsAndMargins result;
+ 
+-  // If that doesn't work, fall back to some hard-coded constants. These will
+-  // apply for app windows, since the app is out of process and the caption
+-  // buttons can't be retrieved. Note that the app window actually positions its
+-  // buttons slightly differently from a standard browser window (it's unclear
+-  // why).
+-  //
+   // These are empirically determined; feel free to change them if they're
+   // not precise.
+   if (@available(macOS 26, *)) {