[presto] Use placeholder cover when scrolling record detail view

presto/main.py

@@ -1997,6 +1997,11 @@ def _draw_detail_cover(app, rec, y):
     if y >= HEIGHT:
         return
 
+    if app.touch.dragging:
+        _draw_placeholder(DETAIL_COVER_X, y,
+                          DETAIL_COVER_SIZE, DETAIL_COVER_SIZE)
+        return
+
     cover_url = _record_detail_cover_url(rec)
 
     if not cover_url:
@@ -2004,12 +2009,7 @@ def _draw_detail_cover(app, rec, y):
                           DETAIL_COVER_SIZE, DETAIL_COVER_SIZE)
         return
 
-    # Use cached data or fetch
     data = rec.get("_detail_thumb_data", None)
-    if data is None and app.touch.dragging:
-        _draw_placeholder(DETAIL_COVER_X, y,
-                          DETAIL_COVER_SIZE, DETAIL_COVER_SIZE)
-        return
 
     if data is None and not rec.get("_detail_thumb_failed", False):
         data = fetch_thumbnail(app, cover_url)

presto/tests/test_screens.py

@@ -96,6 +96,37 @@ class TestSmokeScreens:
         main_module.draw_record_detail(app)
         self._screenshot(main_module, "record-detail")
 
+    def test_record_detail_uses_placeholder_cover_while_dragging(
+        self, main_module, monkeypatch
+    ):
+        """Detail scroll redraws avoid decoding a cached medium JPEG."""
+        app = self._make_app(main_module)
+        app.screen = main_module.STATE_RECORD
+        app.day.selected_day = 15
+        app.touch.dragging = True
+
+        rec = make_mock_record(0)
+        rec["_detail_thumb_data"] = b"cached-jpeg"
+        rec["_detail_thumb_failed"] = False
+        app.day.records = [rec]
+        app.detail.selected_index = 0
+        app.detail.source_screen = main_module.STATE_DAY
+
+        calls = {"jpeg": 0, "placeholder": 0}
+
+        def _count_jpeg(*_args):
+            calls["jpeg"] += 1
+
+        def _count_placeholder(*_args):
+            calls["placeholder"] += 1
+
+        monkeypatch.setattr(main_module, "draw_jpeg", _count_jpeg)
+        monkeypatch.setattr(main_module, "_draw_placeholder", _count_placeholder)
+
+        main_module.draw_record_detail(app)
+
+        assert calls == {"jpeg": 0, "placeholder": 1}
+
     def test_cover_contract_uses_api_sized_images(self, main_module):
         """Cover helpers use the named API sizes expected by the display."""
         rec = make_mock_record(0)