Drop redundant in-frame check

lib/GfxRenderer/GfxRenderer.cpp

@@ -624,7 +624,9 @@ static void renderGlyphFast2BitPortrait(uint8_t* const frameBuffer, const uint8_
                                         const int widthBytes, const int fbOriginY, const int fbRows) {
   for (int glyphX = 0; glyphX < glyphWidth; glyphX++) {
     const int phyY = inverted ? (screenXBase + glyphX) : (displayHeight - 1 - (screenXBase + glyphX));
-    if (phyY < 0 || phyY >= displayHeight) continue;
+    // Single unsigned compare drops both off-band rows (strip mode) and any
+    // out-of-frame row (full-frame mode: fbOriginY=0, fbRows=displayHeight),
+    // matching what the Landscape* cases above do.
     const int rowY = phyY - fbOriginY;
     if (static_cast<unsigned>(rowY) >= static_cast<unsigned>(fbRows)) continue;
     uint8_t* const row = frameBuffer + rowY * widthBytes;
@@ -2656,10 +2658,11 @@ void GfxRenderer::restoreBwBuffer() {
   bwSnapshotRowEnd = 0;
 }
 
-/**
- * Cleanup grayscale buffers using the current frame buffer.
- * Use this when BW buffer was re-rendered instead of stored/restored.
- */
+// Cleanup grayscale buffers using the current frame buffer.
+// Use this when BW buffer was re-rendered instead of stored/restored.
+// On X3 the display call transiently Y-flips frameBuffer in place and flips
+// it back before returning; the logical contents are unchanged but callers
+// must not race a framebuffer reader against this call. See the header.
 void GfxRenderer::cleanupGrayscaleWithFrameBuffer() const {
   if (frameBuffer) {
     display.cleanupGrayscaleBuffers(frameBuffer);

lib/GfxRenderer/GfxRenderer.h

@@ -303,6 +303,18 @@ class GfxRenderer {
   bool storeBwBuffer();                                         // Returns true if buffer was stored successfully
   bool storeBwBufferRect(int x, int y, int width, int height);  // Store only rows intersecting logical rect
   void restoreBwBuffer();                                       // Restore and free the stored buffer
+  // Re-syncs the controller's RED RAM from the current BW framebuffer so the
+  // next differential page turn has a clean baseline. Called after the tiled
+  // grayscale path, which leaves the panel's gray planes loaded but the BW
+  // framebuffer untouched.
+  //
+  // const-correctness caveat: on X3 the underlying display call (see
+  // EInkDisplay::cleanupGrayscaleBuffers) performs an in-place Y-flip of the
+  // framebuffer bytes, sends them, and flips back. The framebuffer's logical
+  // contents are identical before and after, but during the call the bytes
+  // are transiently reordered. The method stays `const` because the renderer's
+  // observable state doesn't change; callers must not race a framebuffer
+  // reader against this call.
   void cleanupGrayscaleWithFrameBuffer() const;
 
   // Font helpers