Cocoa: add event handler to TCocoaConfigApplication, see also !351

lcl/interfaces/cocoa/cocoaconfig.inc

@@ -32,6 +32,14 @@ var
     );
   );
 
+var
+  CocoaConfigApplication: TCocoaConfigApplication = (
+    event: (
+      // highest priority event handler
+      highestHandler: nil;
+    );
+  );
+
 var
   CocoaConfigGlobal: TCocoaConfigGlobal = (
     // for compatiblity with LCL 1.8 release. The macOS base is 72ppi

lcl/interfaces/cocoa/cocoaconfig.pas

@@ -143,6 +143,18 @@ type
     dockMenu: TCocoaConfigDockMenu;
   end;
 
+type
+  // return True for event Handled, bypass post system processing
+  TCocoaApplicationEventHandler = function( event: NSEvent ): Boolean of object;
+
+  TCocoaConfigApplicationEvent = record
+    highestHandler: TCocoaApplicationEventHandler;
+  end;
+
+  TCocoaConfigApplication = record
+    event: TCocoaConfigApplicationEvent;
+  end;
+
 type
   TCocoaConfigGlobal = record
     basePPI: Integer;

lcl/interfaces/cocoa/cocoaint.pas

@@ -666,6 +666,18 @@ begin
     expiration, mode, deqFlag);
   {$endif}
 
+  {
+    Monitors from NSEvent.addLocalMonitorForEvents aren't called until
+    NSApplication.sendEvent, so it's not early enough to capture the tracking
+    run loop behavior below or to prevent key events going into KeyEvBefore, and
+    Application.OnUserInput doesn't allow filtering messages.
+    see also !351
+  }
+  if Assigned(Result) and Assigned(CocoaConfigApplication.event.highestHandler) then begin
+    if CocoaConfigApplication.event.highestHandler(Result) then
+      Exit;
+  end;
+
   if (Result.type_=NSApplicationDefined) and (Result.subtype=LazarusApplicationDefinedSubtypeWakeup) then
     Result:= nil;