Foundation: unmask Windows events on the main RunLoop

When processing events on the main runloop, we should unmask the Windows
message pump events.  If we do not unmask these events, the main RunLoop
is unable to process the UI events and the user has no mechanism for
integrating the Windows message pump events into the run loop.

This is a slightly restrictive compromise to enable the use of RunLoop
on Windows for pumping a UI application.  Because CoreFoundation is not
a supported API surface a user cannot simply invoke the appropriate
CoreFoundation function on the RunLoop's inner (CFRunLoop) object.  In
order to not expose any new CoreFoundation APIs, we need to expose the
functionality from the Swift layer.  This is motivated by the desire to
minimise the divergence from the reference implementation of
CoreFoundation.

The Swift API surface is meant to be near identical to the Objective-C
implementation.  An option would be to expose a SPI for users to control
the behvaiour of the `RunLoop` with regards to the Windows message loop.
One caveat that exists is that CoreFoundation seems to exempt the common
modes from being able to participate in the windows message loop.
Fortunately, the main run loop, which is tied to the main thread, is
configured to run in the default mode which may participate in the OS
message loop.  Additionally, the Windows UI system expects to perform
the operations on the main thread, which gives us a natural point to
alter the behaviour of `RunLoop`.

There are two instances of `CFRunLoopGetMain`, one which provides the
storage for the main runloop, and one which is the accessor for the main
runloop.  Because the accessor will perform additional reference
counting and bridging behavioural changes, use the `CFRunLoop` storage
of the `_mainRunLoop` to acquire the main run loop.

Co-authored-by: Darin Fisher <darinf@gmail.com>

Author: compnerd

Sources/Foundation/RunLoop.swift

@@ -62,9 +62,16 @@ open class RunLoop: NSObject {
         get { unsafeBitCast(_cfRunLoopStorage, to: CFRunLoop?.self) }
         set { _cfRunLoopStorage = newValue }
     }
-    
-    internal static var _mainRunLoop : RunLoop = {
-        return RunLoop(cfObject: CFRunLoopGetMain())
+
+    internal static var _mainRunLoop: RunLoop = {
+        let cfObject = CFRunLoopGetMain()
+#if os(Windows)
+        // Enable the main runloop on Windows to process the Windows UI events.
+        // Windows, similar to AppKit and UIKit, expects to process the UI
+        // events on the main thread.
+        _CFRunLoopSetWindowsMessageQueueMask(cfObject, QS_ALLINPUT, kCFRunLoopDefaultMode)
+#endif
+        return RunLoop(cfObject: cfObject)
     }()
 
     internal init(cfObject : CFRunLoop) {
@@ -76,7 +83,7 @@ open class RunLoop: NSObject {
     }
 
     open class var main: RunLoop {
-        return _CFRunLoopGet2(CFRunLoopGetMain()) as! RunLoop
+        return _CFRunLoopGet2(_mainRunLoop._cfRunLoop) as! RunLoop
     }
 
     open var currentMode: RunLoop.Mode? {