Mimic native focus handling in wxStatusBar on macOS

Improve the logic for determining whether the statusbar should be
renderer as for active/key window or a background one.
wxTopLevelWindow::IsActive() is not a sufficient test because it returns
false in some situations when the statusbar need to appear as active
(another floating window) and also didn't account for window-modal
sheets (which don't change statusbar appearance either).

include/wx/osx/cocoa/private.h

@@ -40,6 +40,7 @@ CGContextRef WXDLLIMPEXP_CORE wxOSXCreateBitmapContextFromNSImage( WX_NSImage ns
 
 wxBitmap WXDLLIMPEXP_CORE wxOSXCreateSystemBitmap(const wxString& id, const wxString &client, const wxSize& size);
 WXWindow WXDLLIMPEXP_CORE wxOSXGetMainWindow();
+WXWindow WXDLLIMPEXP_CORE wxOSXGetKeyWindow();
 
 class WXDLLIMPEXP_FWD_CORE wxDialog;
 

src/osx/carbon/statbrma.cpp

@@ -18,6 +18,7 @@
 #ifndef WX_PRECOMP
     #include "wx/dc.h"
     #include "wx/dcclient.h"
+    #include "wx/dialog.h"
     #include "wx/toplevel.h"
 #endif
 
@@ -147,8 +148,24 @@ void wxStatusBarMac::OnPaint(wxPaintEvent& WXUNUSED(event))
     int w, h;
     GetSize( &w, &h );
 
+    // Notice that wxOSXGetKeyWindow (aka [NSApp keyWindow] used below is
+    // subtly different from IsActive() (aka [NSWindow iskeyWindow]): the
+    // former remains non-NULL if another application shows a temporary
+    // floating window or a status icon's menu is used. That's what we want: in
+    // that case, statusbar appearance shouldn't change. It also shouldn't
+    // change if a window-modal sheet attached to this window is key.
     wxTopLevelWindow *tlw = wxDynamicCast(MacGetTopLevelWindow(), wxTopLevelWindow);
-    if ( tlw && tlw->IsActive() )
+    wxWindow *keyWindow = wxNonOwnedWindow::GetFromWXWindow(wxOSXGetKeyWindow())->MacGetTopLevelWindow();
+    while ( keyWindow && keyWindow != tlw )
+    {
+        wxDialog *dlg = wxDynamicCast(keyWindow, wxDialog);
+        if ( dlg && dlg->GetModality() == wxDIALOG_MODALITY_WINDOW_MODAL )
+            keyWindow = dlg->GetParent();
+        else
+            break;
+    }
+
+    if ( tlw == keyWindow )
     {
         dc.GradientFillLinear(dc.GetSize(), m_bgActiveFrom, m_bgActiveTo, wxBOTTOM);
 

src/osx/carbon/utilscocoa.mm

@@ -345,6 +345,11 @@ WXWindow wxOSXGetMainWindow()
     return [NSApp mainWindow];
 }
 
+WXWindow wxOSXGetKeyWindow()
+{
+    return [NSApp keyWindow];
+}
+
 #endif
 // ----------------------------------------------------------------------------
 // NSImage Utils