Pace frame requests by the frame presented after them

A frame request now only records intent. Each loop step resolves it after
the callbacks that present, so a request made before the present waits for
that frame. An animation that asked first got two REFRESH events per frame
on X11 and with GL.

With no frame in flight, the REFRESH comes one refresh interval after the
last present or answer, so a request without a present cannot spin.

Queued events now go before an owed REFRESH. Input reaches the application
before it draws, and a queued REFRESH pays an owed one for the same surface.

The loop prepare only peeks at a lost event, so deliver can report it as a
final loss. A failed push from another thread wakes the loop.

Signals and posts run at the end of a step, after descriptors and timers.
A signal handler now sees the output that came with it.

A render-thread GL swap counts as in flight on Wayland.

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

1 files changed, 6 insertions, 1 deletions

diff --git a/src/common/backend.h b/src/common/backend.h
index 7633609..a9076e4 100644
--- a/src/common/backend.h
+++ b/src/common/backend.h
@@ -50,10 +50,13 @@ struct yasl_surface {
     struct yasl_private_vector2i reported_size;
     uint32_t reported_scale_120, reported_state;
     bool refresh_owed;
-    /* yasl_surface_request_frame waits for the frame in flight. */
+    /* yasl_surface_request_frame waits for the next frame shown. Each loop
+     * step resolves it after the callbacks that present; see backend.c. */
     bool frame_requested;
     /* Written by GL presents on a rendering thread. */
     _Atomic uint64_t presented_ns;
+    /* When the last frame request was answered. */
+    uint64_t frame_answered_ns;
     yasl_private_timer_id frame_timer;
     void (*refresh)(struct yasl_surface *);
     bool text_input_active;
@@ -93,6 +96,8 @@ struct yasl_surface {
 bool yasl_private_events_open(struct yasl_private_display *d);
 void yasl_private_events_close(struct yasl_private_display *d);
 int yasl_private_events_error(struct yasl_private_display *d);
+/* Whether an event was lost, without reporting it. */
+bool yasl_private_events_failed(struct yasl_private_display *d);
 bool yasl_private_events_pending(struct yasl_private_display *d);
 struct yasl_surface *yasl_private_events_find(struct yasl_private_display *d, uint64_t id);
 bool yasl_private_events_register(struct yasl_surface *s);