fix: give deferred singleShot callbacks a receiver context

Root cause of the long-standing cross-test SystemError flake
("QPushButton returned NULL", CI failing test_title_label after an
unrelated file): TitleLabel schedules
QTimer.singleShot(0, self._apply_collapsed) at construction; when the
banner is deleted before the event loop spins (what every panel test
does), the pending timer fires into the dead C++ object and poisons
whatever test runs next. Deterministic repro added as a regression
test. The receiver-context overload singleShot(0, self, ...) auto-
cancels the callback when the widget dies. Same fix for the two
deferred main-window callbacks (dock split, floating-dock enlarge).

Note for the record: the LOCAL segfault storm this afternoon was the
dev machine's disk hitting 100% full (uv cache at 15G) — Bus errors
from mmap on a full disk, unrelated to any code.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/aare/gui/main_window.py

@@ -2865,7 +2865,7 @@ class MainWindow(QMainWindow):
         if not self._default_dock_split_done:
             self._default_dock_split_done = True
             if not self.state_manager.settings.value("main_window/state"):
-                QTimer.singleShot(0, self._apply_default_dock_split)
+                QTimer.singleShot(0, self, self._apply_default_dock_split)
 
     def _apply_default_dock_split(self) -> None:
         self.resizeDocks(
@@ -3022,7 +3022,7 @@ class MainWindow(QMainWindow):
             # Pop-outs open enlarged instead of keeping the cramped docked
             # size. Deferred: the window is mid-reparent while the signal
             # fires.
-            QTimer.singleShot(0, lambda d=dock: self._enlarge_floating_dock(d))
+            QTimer.singleShot(0, dock, lambda d=dock: self._enlarge_floating_dock(d))
 
     def _enlarge_floating_dock(self, dock: QDockWidget) -> None:
         if not dock.isFloating():

src/aare/gui/widgets/title_label.py

@@ -89,8 +89,12 @@ class TitleLabel(QLabel):
         if settings.value(self._settings_key, default_collapsed, type=bool):
             self._collapsed = True
             # Deferred: the panel adds its other widgets after constructing
-            # the TitleLabel, so siblings don't exist yet.
-            QTimer.singleShot(0, self._apply_collapsed)
+            # the TitleLabel, so siblings don't exist yet. `self` as the
+            # receiver context, or the pending timer outlives a deleted
+            # banner and fires into the dead C++ object — the source of the
+            # long-standing "QPushButton returned NULL" SystemError flake
+            # poisoning whatever test runs next.
+            QTimer.singleShot(0, self, self._apply_collapsed)
 
     def paintEvent(self, event):
         # QSS has no text-shadow, so paint by hand: the QSS background box

tests/unit/gui/test_title_label.py

@@ -73,3 +73,19 @@ def test_not_collapsible_by_default(qtbot):
     title = TitleLabel("Plain", panel)
     grid.addWidget(title, 0, 0)
     assert not hasattr(title, "toggle_button")
+
+
+def test_pending_collapse_timer_dies_with_the_banner():
+    """The deferred _apply_collapsed must not fire into a deleted banner —
+    that was the historic "QPushButton returned NULL" / SystemError flake
+    poisoning whichever test ran next (singleShot without receiver context
+    outlives the widget)."""
+    from PySide6.QtWidgets import QApplication
+
+    _remove_key()
+    host = QWidget()
+    TitleLabel("DoomedBanner", host, collapsible=True, default_collapsed=True)
+    # Python-owned host: del cascades the C++ delete to the banner while
+    # its singleShot(0) is still pending.
+    del host
+    QApplication.processEvents()  # must not raise into the event loop