fix: Apply patch from https://bugzilla.mozilla.org/show_bug.cgi?id=1993474, b=no-bug, c=no-component

src/firefox-patches/bug_1993474_fix.patch

@@ -0,0 +1,30 @@
+diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
+index 2a17159215d107f6e4b09cf3a63ab0fff2c2ee70..622bb36a184a87a1d755b19ff6c3c50027ae53f5 100644
+--- a/widget/windows/nsWindow.cpp
++++ b/widget/windows/nsWindow.cpp
+@@ -2848,6 +2848,25 @@ void nsWindow::SetCustomTitlebar(bool aCustomTitlebar) {
+ 
+   mCustomNonClient = aCustomTitlebar;
+ 
++  // Because of bug 1993474, we want to properly set the WS_BORDER style
++  // when the titlebar is turned on or off.
++  //
++  // The documentation for window styles says that most styles can't be
++  // modified after the window is created. Testing shows that WS_BORDER
++  // seems to be OK, but make sure this is the only style we try to change
++  // here.
++  const LONG_PTR actualStyle = ::GetWindowLongPtrW(mWnd, GWL_STYLE);
++  LONG_PTR newStyle = actualStyle;
++  if (mCustomNonClient) {
++    newStyle &= ~WS_BORDER;
++  } else {
++    newStyle |= WS_BORDER;
++  }
++  if (newStyle != actualStyle) {
++    VERIFY_WINDOW_STYLE(newStyle);
++    ::SetWindowLongPtrW(mWnd, GWL_STYLE, newStyle);
++  }
++  
+   // Force a reflow of content based on the new client dimensions.
+   if (mCustomNonClient) {
+     UpdateNonClientMargins();