Don't flash a blank frame on a sub-frame LCD-off

The emulator pushed a fresh frame the instant the LCD was disabled, and
Display.disableLcd() cleared the buffer and pushed one too. Games that
blank the LCD for only a fraction of a frame - a common way to fit a
large VRAM rewrite in, e.g. the A Bug's Life intro toggles it off for
under a frame every ~9 frames - therefore emitted a full blank frame,
which the display showed as a hard flicker between the picture and a
blank screen (issue #67).

The LCD-off frame is now emitted only after the LCD has stayed off for a
whole frame, at the normal refresh cadence: a brief toggle keeps the
last picture (matching SameBoy and hardware persistence) while a
sustained LCD-off still blanks the screen. disableLcd() clears the
buffer without emitting; the emulation decides when a blank frame is
shown. The A Bug's Life intro now plays the Disney starfield animation
smoothly instead of flickering. Full battery, mealybug and controller
suites stay green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J1wLWscyUGS7CFwc3CjJR8

Author: trekawek

core/src/main/java/eu/rekawek/coffeegb/core/Gameboy.java

@@ -87,6 +87,8 @@ public class Gameboy implements Runnable, Serializable, Originator<Gameboy>, Clo
 
     private boolean lcdDisabled;
 
+    private int lcdOffTicks;
+
     private transient volatile boolean doPause;
 
     private transient volatile boolean paused;
@@ -249,23 +251,34 @@ public boolean tick() {
             hdma.onGpuUpdate(newMode);
         }
 
-        if (!lcdDisabled && !gpu.isLcdEnabled()) {
-            lcdDisabled = true;
-            display.frameIsReady();
-            hdma.onLcdSwitch(false);
-            result = true;
-        } else if (newMode == Mode.VBlank) {
-            requestedScreenRefresh = true;
-            display.frameIsReady();
-            vRamTransfer.frameIsReady();
-            result = true;
-        }
-
-        if (lcdDisabled && gpu.isLcdEnabled()) {
-            lcdDisabled = false;
-            hdma.onLcdSwitch(true);
-        } else if (requestedScreenRefresh && newMode == Mode.OamSearch) {
-            requestedScreenRefresh = false;
+        if (!gpu.isLcdEnabled()) {
+            if (!lcdDisabled) {
+                lcdDisabled = true;
+                hdma.onLcdSwitch(false);
+                lcdOffTicks = 0;
+            }
+            // a game that only blanks the LCD for a fraction of a frame (a common way to
+            // squeeze a big VRAM rewrite in, e.g. the A Bug's Life intro) must not flash a
+            // blank frame - the display keeps the last picture. Only a sustained LCD-off
+            // blanks the screen, at the normal refresh cadence.
+            if (++lcdOffTicks >= TICKS_PER_FRAME) {
+                lcdOffTicks = 0;
+                display.blankFrame();
+                result = true;
+            }
+        } else {
+            if (lcdDisabled) {
+                lcdDisabled = false;
+                hdma.onLcdSwitch(true);
+            }
+            if (newMode == Mode.VBlank) {
+                requestedScreenRefresh = true;
+                display.frameIsReady();
+                vRamTransfer.frameIsReady();
+                result = true;
+            } else if (requestedScreenRefresh && newMode == Mode.OamSearch) {
+                requestedScreenRefresh = false;
+            }
         }
         if (console != null) {
             console.tick();

core/src/main/java/eu/rekawek/coffeegb/core/gpu/Display.java

@@ -6,6 +6,7 @@
 import eu.rekawek.coffeegb.core.memento.Originator;
 
 import java.io.Serializable;
+import java.util.Arrays;
 
 public class Display implements Serializable, Originator<Display> {
 
@@ -62,10 +63,17 @@ public void enableLcd() {
     }
 
     public void disableLcd() {
+        // clear the framebuffer but do not emit a frame here: the emulator decides when
+        // (and whether) a blank frame is shown, so a sub-frame LCD toggle doesn't flicker
         enabled = false;
-        for (i = 0; i < buffer.length; i++) {
-            buffer[i] = 0;
-        }
+        Arrays.fill(buffer, 0);
+        i = 0;
+    }
+
+    /** Emits a blank frame while the LCD stays off (a sustained off blanks the screen). */
+    public void blankFrame() {
+        Arrays.fill(buffer, 0);
+        i = 0;
         frameIsReady();
     }