diff --git a/include/qemu-main.h b/include/qemu-main.h
index 940960a7dbc..2ee83bedff3 100644
--- a/include/qemu-main.h
+++ b/include/qemu-main.h
@@ -5,7 +5,19 @@
 #ifndef QEMU_MAIN_H
 #define QEMU_MAIN_H
 
-int qemu_default_main(void);
+/*
+ * The function to run on the main (initial) thread of the process.
+ * NULL means QEMU's main event loop.
+ * When non-NULL, QEMU's main event loop will run on a purposely created
+ * thread, after which the provided function pointer will be invoked on
+ * the initial thread.
+ * This is useful on platforms which treat the main thread as special
+ * (macOS/Darwin) and/or require all UI API calls to occur from the main
+ * thread. Those platforms can initialise it to a specific function,
+ * while UI implementations may reset it to NULL during their init if they
+ * will handle system and UI events on the main thread via QEMU's own main
+ * event loop.
+ */
 extern int (*qemu_main)(void);
 
 #endif /* QEMU_MAIN_H */
diff --git a/system/main.c b/system/main.c
index 9b91d21ea8c..668d0ecfe8c 100644
--- a/system/main.c
+++ b/system/main.c
@@ -24,26 +24,55 @@
 
 #include "qemu/osdep.h"
 #include "qemu-main.h"
+#include "qemu/main-loop.h"
 #include "sysemu/sysemu.h"
 
 #ifdef CONFIG_SDL
+/*
+ * SDL insists on wrapping the main() function with its own implementation on
+ * some platforms; it does so via a macro that renames our main function, so
+ * <SDL.h> must be #included here even with no SDL code called from this file.
+ */
 #include <SDL.h>
 #endif
 
-int qemu_default_main(void)
+#ifdef CONFIG_DARWIN
+#include <CoreFoundation/CoreFoundation.h>
+#endif
+
+static void *qemu_default_main(void *opaque)
 {
     int status;
 
+    bql_lock();
     status = qemu_main_loop();
     qemu_cleanup(status);
+    bql_unlock();
 
-    return status;
+    exit(status);
 }
 
-int (*qemu_main)(void) = qemu_default_main;
+int (*qemu_main)(void);
+
+#ifdef CONFIG_DARWIN
+static int os_darwin_cfrunloop_main(void)
+{
+    CFRunLoopRun();
+    g_assert_not_reached();
+}
+int (*qemu_main)(void) = os_darwin_cfrunloop_main;
+#endif
 
 int main(int argc, char **argv)
 {
     qemu_init(argc, argv);
-    return qemu_main();
+    bql_unlock();
+    if (qemu_main) {
+        QemuThread main_loop_thread;
+        qemu_thread_create(&main_loop_thread, "qemu_main",
+                           qemu_default_main, NULL, QEMU_THREAD_DETACHED);
+        return qemu_main();
+    } else {
+        qemu_default_main(NULL);
+    }
 }
diff --git a/ui/cocoa.m b/ui/cocoa.m
index dd88115dc6f..0a9ad824aa5 100644
--- a/ui/cocoa.m
+++ b/ui/cocoa.m
@@ -73,6 +73,8 @@
     int height;
 } QEMUScreen;
 
+@class QemuCocoaPasteboardTypeOwner;
+
 static void cocoa_update(DisplayChangeListener *dcl,
                          int x, int y, int w, int h);
 
@@ -107,6 +109,7 @@ static void cocoa_switch(DisplayChangeListener *dcl,
 static NSInteger cbchangecount = -1;
 static QemuClipboardInfo *cbinfo;
 static QemuEvent cbevent;
+static QemuCocoaPasteboardTypeOwner *cbowner;
 
 // Utility functions to run specified code block with the BQL held
 typedef void (^CodeBlock)(void);
@@ -1326,8 +1329,10 @@ - (void) dealloc
 {
     COCOA_DEBUG("QemuCocoaAppController: dealloc\n");
 
-    if (cocoaView)
-        [cocoaView release];
+    [cocoaView release];
+    [cbowner release];
+    cbowner = nil;
+
     [super dealloc];
 }
 
@@ -1943,8 +1948,6 @@ - (void)pasteboard:(NSPasteboard *)sender provideDataForType:(NSPasteboardType)t
 
 @end
 
-static QemuCocoaPasteboardTypeOwner *cbowner;
-
 static void cocoa_clipboard_notify(Notifier *notifier, void *data);
 static void cocoa_clipboard_request(QemuClipboardInfo *info,
                                     QemuClipboardType type);
@@ -2007,43 +2010,8 @@ static void cocoa_clipboard_request(QemuClipboardInfo *info,
     }
 }
 
-/*
- * The startup process for the OSX/Cocoa UI is complicated, because
- * OSX insists that the UI runs on the initial main thread, and so we
- * need to start a second thread which runs the qemu_default_main():
- * in main():
- *  in cocoa_display_init():
- *   assign cocoa_main to qemu_main
- *   create application, menus, etc
- *  in cocoa_main():
- *   create qemu-main thread
- *   enter OSX run loop
- */
-
-static void *call_qemu_main(void *opaque)
-{
-    int status;
-
-    COCOA_DEBUG("Second thread: calling qemu_default_main()\n");
-    bql_lock();
-    status = qemu_default_main();
-    bql_unlock();
-    COCOA_DEBUG("Second thread: qemu_default_main() returned, exiting\n");
-    [cbowner release];
-    exit(status);
-}
-
 static int cocoa_main(void)
 {
-    QemuThread thread;
-
-    COCOA_DEBUG("Entered %s()\n", __func__);
-
-    bql_unlock();
-    qemu_thread_create(&thread, "qemu_main", call_qemu_main,
-                       NULL, QEMU_THREAD_DETACHED);
-
-    // Start the main event loop
     COCOA_DEBUG("Main thread: entering OSX run loop\n");
     [NSApp run];
     COCOA_DEBUG("Main thread: left OSX run loop, which should never happen\n");
@@ -2125,8 +2093,6 @@ static void cocoa_display_init(DisplayState *ds, DisplayOptions *opts)
 
     COCOA_DEBUG("qemu_cocoa: cocoa_display_init\n");
 
-    qemu_main = cocoa_main;
-
     // Pull this console process up to being a fully-fledged graphical
     // app with a menubar and Dock icon
     ProcessSerialNumber psn = { 0, kCurrentProcess };
@@ -2190,6 +2156,12 @@ static void cocoa_display_init(DisplayState *ds, DisplayOptions *opts)
     qemu_clipboard_peer_register(&cbpeer);
 
     [pool release];
+
+    /*
+     * The Cocoa UI will run the NSApplication runloop on the main thread
+     * rather than the default Core Foundation one.
+     */
+    qemu_main = cocoa_main;
 }
 
 static QemuDisplay qemu_display_cocoa = {
diff --git a/ui/gtk.c b/ui/gtk.c
index bf9d3dd679a..c8dcd95350c 100644
--- a/ui/gtk.c
+++ b/ui/gtk.c
@@ -38,6 +38,7 @@
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
 #include "qemu/main-loop.h"
+#include "qemu-main.h"
 
 #include "ui/console.h"
 #include "ui/gtk.h"
@@ -2485,6 +2486,9 @@ static void gtk_display_init(DisplayState *ds, DisplayOptions *opts)
 #ifdef CONFIG_GTK_CLIPBOARD
     gd_clipboard_init(s);
 #endif /* CONFIG_GTK_CLIPBOARD */
+
+    /* GTK's event polling must happen on the main thread. */
+    qemu_main = NULL;
 }
 
 static void early_gtk_display_init(DisplayOptions *opts)
diff --git a/ui/sdl2.c b/ui/sdl2.c
index bd4f5a9da14..44ab2762262 100644
--- a/ui/sdl2.c
+++ b/ui/sdl2.c
@@ -34,6 +34,7 @@
 #include "sysemu/sysemu.h"
 #include "ui/win32-kbd-hook.h"
 #include "qemu/log.h"
+#include "qemu-main.h"
 
 static int sdl2_num_outputs;
 static struct sdl2_console *sdl2_console;
@@ -965,6 +966,9 @@ static void sdl2_display_init(DisplayState *ds, DisplayOptions *o)
     }
 
     atexit(sdl_cleanup);
+
+    /* SDL's event polling (in dpy_refresh) must happen on the main thread. */
+    qemu_main = NULL;
 }
 
 static QemuDisplay qemu_display_sdl2 = {
