qt/wayland: detach subsurface buffer on hide so inactive tabs don't ghost

All panes' wl_subsurfaces share the top-level QWindow's wl_surface
as their parent (commit ef4df3b8d). When a tab switches, Qt
hides the inactive pane's GhosttySurface widgets but the
subsurfaces stayed attached to the top-level with their last
buffers — so the old tab's terminal pixels showed through wherever
the new tab's content didn't cover them.

Add SubsurfacePresenter::hide() which attaches a NULL buffer to
the child surface (= no contribution to compositor frame). Call it
from QEvent::Hide alongside the existing
ghostty_surface_set_occlusion(false) which already throttles
libghostty's renderer thread.

On the next Show + present, the buffer reattaches and the pane
becomes visible again. ghostty_surface_set_occlusion(true) on
Show re-enables the renderer so it produces that first frame.

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

qt/src/GhosttySurface.cpp

@@ -464,6 +464,14 @@ bool GhosttySurface::event(QEvent *e) {
       }
     } else if (e->type() == QEvent::Hide) {
       ghostty_surface_set_occlusion(m_surface, false);
+      // Detach the subsurface buffer so this pane's last frame
+      // doesn't ghost on top of whatever the now-active tab is
+      // showing. The next Show + render reattaches a buffer and
+      // makes it visible again.
+      if (m_subsurfacePresenter) {
+        m_subsurfacePresenter->hide();
+        forceParentCommit();
+      }
     }
   }
   return QWidget::event(e);

qt/src/wayland/SubsurfacePresenter.cpp

@@ -487,4 +487,14 @@ void SubsurfacePresenter::setPosition(int x, int y) {
   wl_display_flush(m_display);
 }
 
+void SubsurfacePresenter::hide() {
+  if (!m_childSurface) return;
+  // Attach NULL = no buffer. After commit + parent commit, the
+  // subsurface contributes nothing to the compositor's frame.
+  // Caller is responsible for forceParentCommit on its side.
+  wl_surface_attach(m_childSurface, nullptr, 0, 0);
+  wl_surface_commit(m_childSurface);
+  wl_display_flush(m_display);
+}
+
 } // namespace wayland

qt/src/wayland/SubsurfacePresenter.h

@@ -134,6 +134,14 @@ public:
   // position to apply. No-op if the position hasn't changed.
   void setPosition(int x, int y);
 
+  // Detach the currently-attached buffer so the subsurface becomes
+  // invisible. Called when the owning GhosttySurface hides (tab
+  // switch) so the inactive pane's pixels don't ghost on top of
+  // whatever the active tab is showing in the same on-screen
+  // region. The next presentDmabuf call re-attaches a buffer and
+  // the subsurface becomes visible again.
+  void hide();
+
   // Called from the wp_fractional_scale_v1.preferred_scale event.
   // Public so the C-style listener struct at file scope in the .cpp
   // can name it; not part of the API for other call sites.