From 1abb1abaf5f03ee87f66946acc3b2fc709a45d73 Mon Sep 17 00:00:00 2001
From: Steven Kirk <grokys@gmail.com>
Date: Wed, 8 Feb 2023 10:42:17 +0100
Subject: [PATCH] Don't overwrite unrelated style mask bits.

Fixes a problem where we were clearing the fullscreen flag erroneously during a fullscreen transition, and in general it's best to preserve flags we're not interested in controlling anyway.
---
 native/Avalonia.Native/src/OSX/WindowImpl.mm | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/native/Avalonia.Native/src/OSX/WindowImpl.mm b/native/Avalonia.Native/src/OSX/WindowImpl.mm
index 4510d135dc..cf1ee6943d 100644
--- a/native/Avalonia.Native/src/OSX/WindowImpl.mm
+++ b/native/Avalonia.Native/src/OSX/WindowImpl.mm
@@ -561,13 +561,15 @@ bool WindowImpl::IsOwned() {
 }
 
 NSWindowStyleMask WindowImpl::CalculateStyleMask() {
-    unsigned long s = NSWindowStyleMaskBorderless;
+    // Use the current style mask and only clear the flags we're going to be modifying.
+    unsigned long s = [Window styleMask] &
+        ~(NSWindowStyleMaskFullSizeContentView |
+          NSWindowStyleMaskTitled |
+          NSWindowStyleMaskClosable |
+          NSWindowStyleMaskResizable |
+          NSWindowStyleMaskMiniaturizable |
+          NSWindowStyleMaskTexturedBackground);
     
-    if(_actualWindowState == FullScreen)
-    {
-        s |= NSWindowStyleMaskFullScreen;
-    }
-
     switch (_decorations) {
         case SystemDecorationsNone:
             s = s | NSWindowStyleMaskFullSizeContentView;