diff --git a/atom.gyp b/atom.gyp
index d3d90dd9c..82e2affb4 100644
--- a/atom.gyp
+++ b/atom.gyp
@@ -89,6 +89,8 @@
       'browser/native_window_mac.h',
       'browser/native_window_mac.mm',
       'browser/native_window_observer.h',
+      'browser/nsalert_synchronous_sheet.h',
+      'browser/nsalert_synchronous_sheet.mm',
       'browser/window_list.cc',
       'browser/window_list.h',
       'browser/window_list_observer.h',
diff --git a/browser/nsalert_synchronous_sheet.h b/browser/nsalert_synchronous_sheet.h
new file mode 100644
index 000000000..5690bbb61
--- /dev/null
+++ b/browser/nsalert_synchronous_sheet.h
@@ -0,0 +1,10 @@
+// Copyright (c) 2013 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import <Cocoa/Cocoa.h>
+
+@interface NSAlert (SynchronousSheet)
+-(NSInteger) runModalSheetForWindow:(NSWindow*)aWindow;
+-(NSInteger) runModalSheet;
+@end
diff --git a/browser/nsalert_synchronous_sheet.mm b/browser/nsalert_synchronous_sheet.mm
new file mode 100644
index 000000000..20e4f489c
--- /dev/null
+++ b/browser/nsalert_synchronous_sheet.mm
@@ -0,0 +1,65 @@
+// Copyright (c) 2013 GitHub, Inc. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+
+#import "browser/nsalert_synchronous_sheet.h"
+
+// Private methods -- use prefixes to avoid collisions with Apple's methods
+@interface NSAlert (SynchronousSheetPrivate)
+-(IBAction) BE_stopSynchronousSheet:(id)sender;   // hide sheet & stop modal
+-(void) BE_beginSheetModalForWindow:(NSWindow *)aWindow;
+@end
+
+@implementation NSAlert (SynchronousSheet)
+
+-(NSInteger) runModalSheetForWindow:(NSWindow *)aWindow {
+    // Set ourselves as the target for button clicks
+    for (NSButton *button in [self buttons]) {
+        [button setTarget:self];
+        [button setAction:@selector(BE_stopSynchronousSheet:)];
+    }
+
+    // Bring up the sheet and wait until stopSynchronousSheet is triggered by a button click
+    [self performSelectorOnMainThread:@selector(BE_beginSheetModalForWindow:) withObject:aWindow waitUntilDone:YES];
+    NSInteger modalCode = [NSApp runModalForWindow:[self window]];
+
+    // This is called only after stopSynchronousSheet is called (that is,
+    // one of the buttons is clicked)
+    [NSApp performSelectorOnMainThread:@selector(endSheet:) withObject:[self window] waitUntilDone:YES];
+
+    // Remove the sheet from the screen
+    [[self window] performSelectorOnMainThread:@selector(orderOut:) withObject:self waitUntilDone:YES];
+
+    return modalCode;
+}
+
+-(NSInteger) runModalSheet {
+    return [self runModalSheetForWindow:[NSApp mainWindow]];
+}
+
+
+#pragma mark Private methods
+
+-(IBAction) BE_stopSynchronousSheet:(id)sender {
+    // See which of the buttons was clicked
+    NSUInteger clickedButtonIndex = [[self buttons] indexOfObject:sender];
+
+    // Be consistent with Apple's documentation (see NSAlert's addButtonWithTitle) so that
+    // the fourth button is numbered NSAlertThirdButtonReturn + 1, and so on
+    NSInteger modalCode = 0;
+    if (clickedButtonIndex == NSAlertFirstButtonReturn)
+        modalCode = NSAlertFirstButtonReturn;
+    else if (clickedButtonIndex == NSAlertSecondButtonReturn)
+        modalCode = NSAlertSecondButtonReturn;
+    else if (clickedButtonIndex == NSAlertThirdButtonReturn)
+        modalCode = NSAlertThirdButtonReturn;
+    else
+        modalCode = NSAlertThirdButtonReturn + (clickedButtonIndex - 2);
+
+    [NSApp stopModalWithCode:modalCode];
+}
+
+-(void) BE_beginSheetModalForWindow:(NSWindow *)aWindow {
+    [self beginSheetModalForWindow:aWindow modalDelegate:nil didEndSelector:nil contextInfo:nil];
+}
+
+@end