When we call mg_surface_prepare() on a surface, select it in a per-thread handle. Automatically call deselect on the selected handle when preparing another surface. Also have a way to deselect whichever surface is currently selected. This is needed for some backend that explicitly need to be 'unbound' before using them from another thread

src/graphics.h

@@ -99,8 +99,11 @@ MP_API bool mg_surface_is_nil(mg_surface surface);
 
 MP_API mg_surface mg_surface_create_for_window(mp_window window, mg_surface_api api);
 MP_API void mg_surface_destroy(mg_surface surface);
+
 MP_API void mg_surface_prepare(mg_surface surface);
 MP_API void mg_surface_present(mg_surface surface);
+MP_API void mg_surface_deselect(void);
+
 MP_API void mg_surface_swap_interval(mg_surface surface, int swap);
 MP_API vec2 mg_surface_contents_scaling(mg_surface surface);
 MP_API mp_rect mg_surface_get_frame(mg_surface surface);

src/graphics_surface.c

@@ -8,6 +8,12 @@
 
 #include"graphics_surface.h"
 
+//---------------------------------------------------------------
+// per-thread selected surface
+//---------------------------------------------------------------
+
+mp_thread_local mg_surface __mgSelectedSurface = {0};
+
 //---------------------------------------------------------------
 // typed handles functions
 //---------------------------------------------------------------
@@ -208,13 +214,32 @@ void mg_surface_destroy(mg_surface handle)
 	}
 }
 
+void mg_surface_deselect()
+{
+	DEBUG_ASSERT(__mgData.init);
+
+	mg_surface_data* prevSurface = mg_surface_data_from_handle(__mgSelectedSurface);
+	if(prevSurface && prevSurface->deselect)
+	{
+		prevSurface->deselect(prevSurface);
+	}
+	__mgSelectedSurface = mg_surface_nil();
+}
+
 void mg_surface_prepare(mg_surface surface)
 {
 	DEBUG_ASSERT(__mgData.init);
+
+	if(surface.h != __mgSelectedSurface.h)
+	{
+		mg_surface_deselect();
+	}
+
 	mg_surface_data* surfaceData = mg_surface_data_from_handle(surface);
 	if(surfaceData && surfaceData->prepare)
 	{
 		surfaceData->prepare(surfaceData);
+		__mgSelectedSurface = surface;
 	}
 }
 

src/graphics_surface.h

@@ -23,6 +23,7 @@ typedef struct mg_canvas_backend mg_canvas_backend;
 
 typedef void (*mg_surface_destroy_proc)(mg_surface_data* surface);
 typedef void (*mg_surface_prepare_proc)(mg_surface_data* surface);
+typedef void (*mg_surface_deselect_proc)(mg_surface_data* surface);
 typedef void (*mg_surface_present_proc)(mg_surface_data* surface);
 typedef void (*mg_surface_swap_interval_proc)(mg_surface_data* surface, int swap);
 typedef vec2 (*mg_surface_contents_scaling_proc)(mg_surface_data* surface);
@@ -42,6 +43,7 @@ typedef struct mg_surface_data
 	mg_surface_destroy_proc destroy;
 	mg_surface_prepare_proc prepare;
 	mg_surface_present_proc present;
+	mg_surface_deselect_proc deselect;
 	mg_surface_swap_interval_proc swapInterval;
 	mg_surface_contents_scaling_proc contentsScaling;
 	mg_surface_get_frame_proc getFrame;

src/mtl_surface.m

@@ -158,6 +158,7 @@ mg_surface_data* mg_mtl_surface_create_for_window(mp_window window)
 		surface->interface.api = MG_METAL;
 		surface->interface.destroy = mg_mtl_surface_destroy;
 		surface->interface.prepare = mg_mtl_surface_prepare;
+		surface->interface.deselect = 0;
 		surface->interface.present = mg_mtl_surface_present;
 		surface->interface.swapInterval = mg_mtl_surface_swap_interval;
 

src/wgl_surface.c

@@ -153,6 +153,11 @@ void mg_wgl_surface_present(mg_surface_data* interface)
 	SwapBuffers(surface->hDC);
 }
 
+void mg_wgl_surface_deselect(mg_surface_data* interface)
+{
+	wglMakeCurrent(NULL, NULL);
+}
+
 void mg_wgl_surface_swap_interval(mg_surface_data* interface, int swap)
 {
 	mg_wgl_surface* surface = (mg_wgl_surface*)interface;