Fix Qt indexing in `GapAnnotations.reposition()`

`prepareGeometryChange()` ran after the gap rects and arrow specs
were mutated. Qt could retain the item's old scene index even
though `boundingRect()` returned its new prepended-history
position.

Invalidate cached bounds and notify Qt before changing any
geometry. Cover both scene lookup positions with a real offscreen
item.

(this patch was generated in some part by `opencode` using `gpt-5.6-sol` (`openai`))

piker/ui/_annotate.py

@@ -670,6 +670,11 @@ class GapAnnotations(GraphicsObject):
                 )
             }
 
+        # Notify Qt before mutating anything used by
+        # `boundingRect()`.
+        self.prepareGeometryChange()
+        self._br = None
+
         # rebuild rect array from gap specs with new indices
         rect_memory = self._rectarray.ndarray()
 
@@ -777,7 +782,4 @@ class GapAnnotations(GraphicsObject):
             self._arrow_path.addPolygon(arrow_poly)
             self._arrow_path.closeSubpath()
 
-        # invalidate bounding rect cache
-        self._br = None
-        self.prepareGeometryChange()
         self.update()

tests/test_gap_overlays.py

@@ -10,6 +10,7 @@ os.environ['QT_QPA_PLATFORM'] = 'offscreen'
 
 import msgspec
 import numpy as np
+from PyQt6.QtWidgets import QGraphicsScene
 import pyqtgraph as pg
 import pytest
 
@@ -22,7 +23,11 @@ from piker.ui._gaps import (
     SetGapOverlay,
     gap_specs_from_ohlcv,
 )
-from piker.ui.qt import QApplication
+from piker.ui.qt import (
+    QApplication,
+    QPointF,
+    QRectF,
+)
 
 
 FQME: str = 'gap.test'
@@ -387,6 +392,79 @@ def test_gap_manager_real_qt_lifecycle(
         qapp.processEvents()
 
 
+def test_gap_annotations_reposition_after_prepend(
+    qapp: QApplication,
+
+) -> None:
+    '''
+    Reposition live gap geometry after history index offsets change.
+
+    A history prepend preserves timestamps but shifts every absolute
+    chart index. Gap rectangles and arrows otherwise remain attached
+    to their pre-prepend x coordinates. Render one real gap item,
+    shift its source indexes by ten, call `reposition()`, and prove
+    its rectangle bounds and arrow spec move together while the item
+    stays attached to its real Qt scene.
+
+    '''
+    ds: SimpleNamespace
+    charts: tuple[_ChartStub, _ChartStub]
+    ds, charts = _display_state(FQME)
+    annots: dict[int, GapAnnotations] = {}
+    gapman: GapOverlayMngr = GapOverlayMngr(
+        dss={FQME: ds},
+        annots=annots,
+    )
+
+    try:
+        gap: GapOverlay = gapman.refresh(
+            ds=ds,
+            timeframe=60,
+        )
+        assert gap.aid is not None
+        item: GapAnnotations = annots[gap.aid]
+        old_br: QRectF = item.boundingRect()
+        scene: QGraphicsScene = charts[1].widget.scene()
+        old_scene_point: QPointF = item.mapToScene(
+            old_br.center()
+        )
+        qapp.processEvents()
+        assert item in scene.items(old_scene_point)
+        old_rects: np.ndarray = (
+            item._rectarray.ndarray().copy()
+        )
+        shifted: np.ndarray = ds.hist_viz.shm.array.copy()
+        shifted['index'] += 10
+
+        item.reposition(
+            array=shifted,
+            fqme=FQME,
+            timeframe=60,
+        )
+        qapp.processEvents()
+
+        new_rects: np.ndarray = item._rectarray.ndarray()
+        np.testing.assert_allclose(
+            new_rects[:, 0],
+            old_rects[:, 0] + 10,
+        )
+        assert item._gap_specs[0]['arrow_x'] == 12
+        new_br: QRectF = item.boundingRect()
+        new_scene_point: QPointF = item.mapToScene(
+            new_br.center()
+        )
+        assert new_br.left() == old_br.left() + 10
+        assert item in scene.items(new_scene_point)
+        assert item not in scene.items(old_scene_point)
+        assert item.scene() is charts[1].widget.scene()
+    finally:
+        gapman.remove_owner('chart-local')
+        chart: _ChartStub
+        for chart in charts:
+            chart.close()
+        qapp.processEvents()
+
+
 def test_duplicate_fqme_layers_use_local_chart_identity(
     qapp: QApplication,