Turns out the last Cocoa fix was neither reliable or actually working.

Cocoa/Document.m

@@ -334,8 +334,6 @@ static void printImage(GB_gameboy_t *gb, uint32_t *image, uint8_t height,
     NSMutableParagraphStyle *paragraph_style = [[NSMutableParagraphStyle alloc] init];
     [paragraph_style setLineSpacing:2];
         
-    self.mainWindow.contentView.layer.backgroundColor = [[NSColor blackColor] CGColor];
-    
     self.debuggerSideViewInput.font = [NSFont userFixedPitchFontOfSize:12];
     self.debuggerSideViewInput.textColor = [NSColor whiteColor];
     self.debuggerSideViewInput.defaultParagraphStyle = paragraph_style;

Cocoa/Document.xib

@@ -49,12 +49,18 @@
             <view key="contentView" id="gIp-Ho-8D9">
                 <rect key="frame" x="0.0" y="0.0" width="160" height="144"/>
                 <autoresizingMask key="autoresizingMask"/>
+                <subviews>
+                    <customView id="KTk-4M-J7t" customClass="GBBorderView">
+                        <rect key="frame" x="0.0" y="0.0" width="160" height="144"/>
+                        <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                         <subviews>
                             <view id="uqf-pe-VAF" customClass="GBView">
                                 <rect key="frame" x="0.0" y="0.0" width="160" height="144"/>
                                 <autoresizingMask key="autoresizingMask" widthSizable="YES" heightSizable="YES"/>
                             </view>
                         </subviews>
+                    </customView>
+                </subviews>
             </view>
             <connections>
                 <outlet property="delegate" destination="-2" id="jvM-lC-fKb"/>

Cocoa/GBBorderView.h

@@ -0,0 +1,5 @@
+#import <Cocoa/Cocoa.h>
+
+@interface GBBorderView : NSView
+
+@end

Cocoa/GBBorderView.m

@@ -0,0 +1,26 @@
+#import "GBBorderView.h"
+
+@implementation GBBorderView
+
+
+- (void)awakeFromNib
+{
+    self.wantsLayer = YES;
+}
+
+- (BOOL)wantsUpdateLayer
+{
+    return YES;
+}
+
+- (void)updateLayer
+{
+    /* Wonderful, wonderful windowserver(?) bug. Using 0,0,0 here would cause it to render garbage
+       on fullscreen windows on some High Sierra machines. Any other value, including the one used
+       here (which is rendered exactly the same due to rounding) works around this bug. */
+    self.layer.backgroundColor = [NSColor colorWithCalibratedRed:0
+                                                           green:0
+                                                            blue:1.0 / 1024.0
+                                                           alpha:1.0].CGColor;
+}
+@end

Cocoa/GBViewMetal.m

@@ -71,13 +71,6 @@ static const vector_float2 rect[] =
     [self loadShader];
 }
 
-- (void)addSubview:(NSView *)view
-{
-    /* Fixes rounded corners */
-    [super addSubview:view];
-    [self setWantsLayer:YES];
-}
-
 - (void) loadShader
 {
     NSError *error = nil;