Read back the last presented CPU frame

wsap_term writes PNG screenshots for its debug server and keeps its own
copy of the frame because YASL had no readback.

yasl_surface_read_pixels() copies the frame from the last successful
present, or reports its extent when the buffer is too small. X11 and
Cocoa keep it in their single CPU target until the next draw starts;
X11 now also forgets it when a resize replaces the target, and Cocoa
records the presented extent because a resize can change the size before
the next draw. Wayland remembers which buffer it committed and forgets it
when a draw reuses that buffer.

The backend test checks the copy on Wayland and X11, and that draws that
never present do not show through. The Cocoa path is not built or run
here.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/include/yasl/yasl.h b/include/yasl/yasl.h
index 7c9bfbd..5946816 100644
--- a/include/yasl/yasl.h
+++ b/include/yasl/yasl.h
@@ -777,6 +777,25 @@ YASL_API bool yasl_surface_present(struct yasl_surface *s, const struct yasl_fra
  */
 YASL_API bool yasl_surface_poll_event(struct yasl_surface *s, struct yasl_event *ev);
 /**
+ * Copy the last frame presented from the surface's CPU target.
+ * @param s Surface.
+ * @param out Output for size->x * size->y premultiplied ARGB8888 pixels with
+ * tightly packed rows, or NULL when cap is zero.
+ * @param cap Capacity of out in pixels.
+ * @param size Required output: the frame extent in device pixels, or zero when
+ * no frame is available.
+ * @return true if copied. false leaves out unchanged; when *size is nonzero,
+ * out was too small, so allocate size->x * size->y pixels and call again.
+ * A frame is available from a successful yasl_surface_present() until the
+ * next draw callback starts, and again once that draw presents. After a draw
+ * that does not present or a resize that replaces the target, no frame may be
+ * available; native rendering never has one. Call outside draw callbacks. This copies YASL's frame, not
+ * the composited screen: decorations, other windows, and color conversion are
+ * not included.
+ */
+YASL_API bool yasl_surface_read_pixels(struct yasl_surface *s, uint32_t *out, size_t cap,
+                                      struct yasl_vector2i *size);
+/**
  * Select absolute or relative (locked) pointer input for a surface.
  * @param s Surface.
  * @param mode YASL_POINTER_NORMAL or YASL_POINTER_RELATIVE.