feat: scrolling

native/include/lua/providers.h

@@ -137,6 +137,36 @@ public:
   // clean up ghosting all stay here. A live LCD has nothing pending and does
   // nothing.
   virtual void commit() = 0;
+  // Offscreen band for composited repaints. beginBuffer opens a RAM surface
+  // covering the screen rectangle (x, y, w, h); every subsequent draw keeps its
+  // screen coordinates (the driver translates into the band) and clips to it,
+  // so a caller re-walks the same tree per band without renumbering anything.
+  // present() blits the band back to (x, y) and returns drawing to the panel.
+  // Re-rasterizing into RAM and pushing once collapses a per-primitive bus
+  // transaction storm into a streamed write, which is what direct-to-panel
+  // drawing cannot make smooth. A driver with no spare RAM (or an e-ink panel
+  // that gains nothing) refuses the band, so the painter draws to the panel.
+  virtual bool beginBuffer(int32_t x, int32_t y, int32_t w, int32_t h) {
+    (void)x;
+    (void)y;
+    (void)w;
+    (void)h;
+    return false;
+  }
+  virtual void present() {}
+  // Clips every draw to this screen rectangle until clearClip(). The painter
+  // wraps a custom node in its own box, because a node owns its box and nothing
+  // else's: a band is usually taller than the node it covers, so a painter that
+  // draws outside itself -- a list row half scrolled off the top -- would
+  // otherwise land on whatever else that band covers. Not clipping is only safe
+  // while no custom node ever overdraws.
+  virtual void setClip(int32_t x, int32_t y, int32_t w, int32_t h) {
+    (void)x;
+    (void)y;
+    (void)w;
+    (void)h;
+  }
+  virtual void clearClip() {}
   virtual void fillPolygon(const int32_t* xs, const int32_t* ys, size_t count,
                            int32_t color) = 0;
   // Null coordinates centre the image; null bounds fall back to the panel size.