Don't use extra margins around content of wxStaticBox in wxOSX.

This makes top and left borders for the controls inside the box the same as
right and bottom ones.

Closes #16808.

(this is a backport of bd177b0635 from master)

docs/changes.txt

@@ -643,6 +643,7 @@ wxOSX:
 - Generate wxEVT_TEXT_ENTER for wxTE_PASSWORD controls too (mj_smoker).
 - Send wxIconizeEvent when a window is iconized/restore (Rob Krakora).
 - Use correct colour for disabled wxStaticText (sbrowne).
+- Fix too large top and left margins inside wxStaticBox (sbrowne).
 - Fix bottom margins sizes for several controls (sbrowne).
 - Fix initial position of controls with layout insets (Tim Kosse).
 - Don't allow pasting rich text in non-wxTE_RICH text controls (Tim Kosse).

src/osx/cocoa/statbox.mm

@@ -71,6 +71,9 @@ wxWidgetImplType* wxWidgetImpl::CreateGroupBox( wxWindowMac* wxpeer,
 {
     NSRect r = wxOSXGetFrameForControl( wxpeer, pos , size ) ;
     wxNSBox* v = [[wxNSBox alloc] initWithFrame:r];
+    NSSize margin = { 0.0, 0.0 };
+    [v setContentViewMargins: margin];
+    [v sizeToFit];
     wxStaticBoxCocoaImpl* c = new wxStaticBoxCocoaImpl( wxpeer, v );
 #if !wxOSX_USE_NATIVE_FLIPPED
     c->SetFlipped(false);