GBHawk: properly fade to white in GBC mode

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBC_GB_PPU.cs

@@ -130,6 +130,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 						OAM_access_write = true;
 
 						clear_screen = true;
+						Core.clear_counter = 0;
 
 						// turing off the screen causes HDMA to run for one cycle
 						HDMA_run_once = true;
@@ -139,6 +140,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 					{
 						// don't draw for one frame after turning on
 						blank_frame = true;
+						clear_screen = false;
 					}
 					LCDC = value;
 					break; 

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBC_PPU.cs

@@ -130,12 +130,16 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 						// turing off the screen causes HDMA to run for one cycle
 						HDMA_run_once = true;
+
+						clear_screen = true;
+						Core.clear_counter = 0;
 					}
 
 					if (!LCDC.Bit(7) && value.Bit(7))
 					{
 						// don't draw for one frame after turning on
 						blank_frame = true;
+						clear_screen = false;
 					}
 
 					LCDC = value;

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IEmulator.cs

@@ -163,12 +163,24 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 
 			// turn off the screen so the image doesnt persist
 			// but don't turn off blank_frame yet, it still needs to be true until the next VBL
-			// this doesn't run for GBC, some games, ex MIB the series 2, rely on the screens persistence while off to make video look smooth.
+			// GBC games need to clear slow enough that games that turn the screen off briefly for cinematics still look smooth
-			// But some GB gams, ex Battletoads, turn off the screen for a long time from the middle of the frame, so need to be cleared.
 			if (ppu.clear_screen)
 			{
-				for (int j = 0; j < frame_buffer.Length; j++) { frame_buffer[j] = (int)ppu.color_palette[0]; }
+				if (is_GBC)
-				ppu.clear_screen = false;
+				{
+					for (int j = 0; j < frame_buffer.Length; j++) { frame_buffer[j] = (int)(frame_buffer[j] | (0x30303 << (clear_counter * 2))); }
+
+					clear_counter++;
+					if (clear_counter == 4)
+					{
+						ppu.clear_screen = false;
+					}				
+				}
+				else
+				{
+					for (int j = 0; j < frame_buffer.Length; j++) { frame_buffer[j] = (int)ppu.color_palette[0]; }
+					ppu.clear_screen = false;
+				}
 			}
 		}
 

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.IStatable.cs

@@ -32,6 +32,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			ser.Sync(nameof(delays_to_process), ref delays_to_process);
 			ser.Sync(nameof(controller_delay_cd), ref controller_delay_cd);
 			ser.Sync(nameof(cpu_state_hold), ref cpu_state_hold);
+			ser.Sync(nameof(clear_counter), ref clear_counter);
 
 			ser.Sync(nameof(REG_FFFF), ref REG_FFFF);
 			ser.Sync(nameof(REG_FF0F), ref REG_FF0F);

src/BizHawk.Emulation.Cores/Consoles/Nintendo/GBHawk/GBHawk.cs

@@ -328,6 +328,7 @@ namespace BizHawk.Emulation.Cores.Nintendo.GBHawk
 			RAM_Bank = 1; // RAM bank always starts as 1 (even writing zero still sets 1)
 			delays_to_process = false;
 			controller_delay_cd = 0;
+			clear_counter = 0;
 
 			Register_Reset();
 			timer.Reset();