qt/wayland: subsurface in sync mode for atomic-with-parent resize

The previous desync mode meant our wl_subsurface commits applied
immediately, independent of Qt's parent surface commits. That's
lower-latency in the steady state but during resize it left a
one-frame window where the parent had already grown to the new
size but our child subsurface was still showing its old-size
buffer — the parent's translucent QWidget background showed
through the gap. The original pre-subsurface QPainter blit didn't
have this issue because everything was on one surface and resize
was inherently atomic.

Sync mode (the wl_subsurface default) restores that atomicity:
our wl_surface.commit on the child caches state until the parent
commits, then both apply in the same compositor frame. Resize
becomes lockstep.

The cost is that frames now need a parent commit to become
visible. drainVulkan now calls update() after each presentDmabuf
to schedule the paintEvent that triggers Qt's backing-store
flush (= parent wl_surface.commit). Latency penalty vs desync:
one event-loop turn (sub-millisecond at idle).

Pairs with b8d2f25cf (synchronous draw in resizeEvent) — that
fix attached the new-size buffer to the subsurface inside
resizeEvent; this fix ensures the attach gets applied atomically
with the parent's next commit.

Co-Authored-By: claude-flow <ruv@ruv.net>

qt/src/GhosttySurface.cpp

@@ -1551,6 +1551,12 @@ void GhosttySurface::drainVulkan() {
     m_subsurfacePresenter->presentDmabuf(
         frame.fd, frame.drm_format, frame.drm_modifier, frame.width,
         frame.height, frame.stride, width(), height());
+    // The subsurface is in wl_subsurface sync mode, so the buffer
+    // we just attached only becomes visible when Qt's parent surface
+    // commits. update() schedules a paintEvent which triggers
+    // Qt's backing-store flush (= parent wl_surface.commit), at
+    // which point our cached subsurface state applies atomically.
+    update();
     return;
   }
 

qt/src/wayland/SubsurfacePresenter.cpp

@@ -254,11 +254,20 @@ SubsurfacePresenter::tryCreate(QWindow *parent) {
     return nullptr;
   }
 
-  // Independent frame pacing: the renderer's present cadence is
-  // driven by libghostty's render thread, not the GUI thread's paint
-  // cycle, so we don't want our wl_subsurface state changes to wait
-  // for the parent's next commit. `set_desync` is what allows that.
-  wl_subsurface_set_desync(sub);
+  // Sync mode (the wl_subsurface default — we don't call set_desync).
+  // In sync mode our wl_surface.commit caches state until the parent
+  // surface commits, at which point both apply atomically. That's
+  // what gives resize its lockstep behavior — parent grows to the
+  // new size and our subsurface's matching new-size buffer apply in
+  // the same compositor frame, so there's no transient gap where the
+  // parent's translucent background shows through.
+  //
+  // The cost: our frames need a parent commit to become visible. The
+  // GhosttySurface caller compensates by calling update() after each
+  // presentDmabuf — that schedules a paintEvent, which triggers Qt
+  // to flush the parent surface's backing store (= a wl_surface.commit
+  // on the parent). Total latency penalty vs desync: one event-loop
+  // turn, sub-millisecond at idle.
 
   // Subsurface covers the parent at the origin. Phase 4 will keep
   // this in sync on splits/tabs/etc.; for now the GhosttySurface