create a sizing callback for the preview panel specifically, apparently on macos the window size isn't guaranteed to be the size it's set to in the main window size handler

libobs/obs.c

@@ -425,7 +425,7 @@ bool obs_enum_output_types(size_t idx, const char **id)
 
 graphics_t obs_graphics(void)
 {
-	return obs->video.graphics;
+	return (obs != NULL) ? obs->video.graphics : NULL;
 }
 
 media_t obs_media(void)

obs/forms/OBSWindows.cpp

@@ -240,6 +240,7 @@ OBSBasicBase::OBSBasicBase( wxWindow* parent, wxWindowID id, const wxString& tit
 	this->Connect( fileOpenMenu->GetId(), wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileOpenClicked ) );
 	this->Connect( fileSaveMenu->GetId(), wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileSaveClicked ) );
 	this->Connect( fileExitMenu->GetId(), wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileExitClicked ) );
+	previewPanel->Connect( wxEVT_SIZE, wxSizeEventHandler( OBSBasicBase::OnResizePreview ), NULL, this );
 	scenes->Connect( wxEVT_COMMAND_LISTBOX_SELECTED, wxCommandEventHandler( OBSBasicBase::scenesClicked ), NULL, this );
 	scenes->Connect( wxEVT_RIGHT_DOWN, wxMouseEventHandler( OBSBasicBase::scenesRDown ), NULL, this );
 	this->Connect( ID_SCENE_ADD, wxEVT_COMMAND_TOOL_CLICKED, wxCommandEventHandler( OBSBasicBase::sceneAddClicked ) );
@@ -271,6 +272,7 @@ OBSBasicBase::~OBSBasicBase()
 	this->Disconnect( IF_FILE_OPEN, wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileOpenClicked ) );
 	this->Disconnect( IF_FILE_SAVE, wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileSaveClicked ) );
 	this->Disconnect( ID_FILE_EXIT, wxEVT_COMMAND_MENU_SELECTED, wxCommandEventHandler( OBSBasicBase::fileExitClicked ) );
+	previewPanel->Disconnect( wxEVT_SIZE, wxSizeEventHandler( OBSBasicBase::OnResizePreview ), NULL, this );
 	scenes->Disconnect( wxEVT_COMMAND_LISTBOX_SELECTED, wxCommandEventHandler( OBSBasicBase::scenesClicked ), NULL, this );
 	scenes->Disconnect( wxEVT_RIGHT_DOWN, wxMouseEventHandler( OBSBasicBase::scenesRDown ), NULL, this );
 	this->Disconnect( ID_SCENE_ADD, wxEVT_COMMAND_TOOL_CLICKED, wxCommandEventHandler( OBSBasicBase::sceneAddClicked ) );

obs/forms/OBSWindows.fbp

@@ -394,7 +394,7 @@
                                                             <event name="OnRightDown"></event>
                                                             <event name="OnRightUp"></event>
                                                             <event name="OnSetFocus"></event>
-                                                            <event name="OnSize"></event>
+                                                            <event name="OnSize">OnResizePreview</event>
                                                             <event name="OnUpdateUI"></event>
                                                         </object>
                                                     </object>

obs/forms/OBSWindows.h

@@ -139,6 +139,7 @@ class OBSBasicBase : public WindowSubclass
 		virtual void fileOpenClicked( wxCommandEvent& event ) { event.Skip(); }
 		virtual void fileSaveClicked( wxCommandEvent& event ) { event.Skip(); }
 		virtual void fileExitClicked( wxCommandEvent& event ) { event.Skip(); }
+		virtual void OnResizePreview( wxSizeEvent& event ) { event.Skip(); }
 		virtual void scenesClicked( wxCommandEvent& event ) { event.Skip(); }
 		virtual void scenesRDown( wxMouseEvent& event ) { event.Skip(); }
 		virtual void sceneAddClicked( wxCommandEvent& event ) { event.Skip(); }

obs/window-basic-main.cpp

@@ -185,11 +185,20 @@ void OBSBasic::OnSize(wxSizeEvent &event)
 		return;
 
 	ResizePreview(ovi.base_width, ovi.base_height);
+}
+
+void OBSBasic::OnResizePreview(wxSizeEvent &event)
+{
+	event.Skip();
+
 	wxSize newSize = previewPanel->GetMinSize();
 
-	gs_entercontext(obs_graphics());
-	gs_resize(newSize.x, newSize.y);
-	gs_leavecontext();
+	graphics_t graphics = obs_graphics();
+	if (graphics) {
+		gs_entercontext(graphics);
+		gs_resize(newSize.x, newSize.y);
+		gs_leavecontext();
+	}
 }
 
 void OBSBasic::fileNewClicked(wxCommandEvent &event)

obs/window-basic-main.hpp

@@ -43,6 +43,7 @@ protected:
 	virtual void OnClose(wxCloseEvent &event);
 	virtual void OnMinimize(wxIconizeEvent &event);
 	virtual void OnSize(wxSizeEvent &event);
+	virtual void OnResizePreview(wxSizeEvent &event);
 	virtual void fileNewClicked(wxCommandEvent &event);
 	virtual void fileOpenClicked(wxCommandEvent &event);
 	virtual void fileSaveClicked(wxCommandEvent &event);