Fix UI freeze on first cover load (decode+scale off the GUI thread)

app/gui.py

@@ -117,14 +117,16 @@ def _thread_running(thread):
 
 
 class CoverLoader(QObject):
-    loaded = Signal(int, int, str)  # gen, item index, cached image path
+    loaded = Signal(int, int, QImage)  # gen, item index, fitted cover image
     failed = Signal(int, int, str)  # gen, item index, name (no cover found)
     finished = Signal()
 
-    def __init__(self, items, gen):
+    def __init__(self, items, gen, icon_w=150, icon_h=220):
         super().__init__()
         self.items = items
         self.gen = gen
+        self.icon_w = icon_w
+        self.icon_h = icon_h
         self._cancel = threading.Event()
 
     def cancel(self):
@@ -148,7 +150,11 @@ class CoverLoader(QObject):
                 except Exception:  # noqa: BLE001
                     path = None
                 if path is not None:
-                    self.loaded.emit(self.gen, i, str(path))
+                    img = QImage(str(path))
+                    if img.isNull():
+                        self.failed.emit(self.gen, i, self.items[i]["name"])
+                    else:
+                        self.loaded.emit(self.gen, i, _fit_image(img, self.icon_w, self.icon_h))
                 else:
                     self.failed.emit(self.gen, i, self.items[i]["name"])
         finally:
@@ -276,6 +282,25 @@ def _fit_pixmap(pm, w, h):
     return canvas
 
 
+def _fit_image(img, w, h):
+    """Scale a QImage into a letterboxed ``w x h`` ARGB image (thread-safe).
+
+    Decoding + scaling happen off the GUI thread in the cover loaders; the
+    main thread only converts the result to a QPixmap (cheap).
+    """
+    canvas = QImage(w, h, QImage.Format_ARGB32)
+    canvas.fill(Qt.transparent)
+    if img.isNull():
+        return canvas
+    scaled = img.scaled(w, h, Qt.KeepAspectRatio, Qt.SmoothTransformation)
+    x = (w - scaled.width()) // 2
+    y = (h - scaled.height()) // 2
+    painter = QPainter(canvas)
+    painter.drawImage(x, y, scaled)
+    painter.end()
+    return canvas
+
+
 def _cover_pixmap(rid, system, max_w, max_h):
     """Return a scaled cover pixmap for an item, or None if unavailable."""
     if rid is None:
@@ -429,7 +454,7 @@ class CoverShowcase(QWidget):
         for loader in self._loaders:
             loader.cancel()
         self.thread = QThread(self)
-        self.loader = CoverLoader(self.items, gen)
+        self.loader = CoverLoader(self.items, gen, self.ICON_W, self.ICON_H)
         self.loader.moveToThread(self.thread)
         self.thread.started.connect(self.loader.run)
         self.loader.loaded.connect(self._on_loaded)
@@ -454,12 +479,12 @@ class CoverShowcase(QWidget):
         painter.end()
         return QIcon(pm)
 
-    def _on_loaded(self, gen, index, path):
+    def _on_loaded(self, gen, index, image):
         if gen != self._generation:
             return
         item = self.list.item(index)
         if item is not None:
-            item.setIcon(self._fit_icon(QPixmap(path)))
+            item.setIcon(QIcon(QPixmap.fromImage(image)))
         if 0 <= index < len(self.items):
             self.cover_loaded.emit(self.items[index])
 
@@ -1479,14 +1504,16 @@ class StoreScanWorker(QObject):
 
 
 class StoreCoverLoader(QObject):
-    loaded = Signal(int, int, str)  # gen, grid row, cached image path
+    loaded = Signal(int, int, QImage)  # gen, grid row, fitted cover image
     finished = Signal()
 
-    def __init__(self, tasks, gen, quick=True):
+    def __init__(self, tasks, gen, quick=True, icon_w=150, icon_h=220):
         super().__init__()
         self.tasks = tasks  # list of (grid_row, item_dict)
         self.gen = gen
         self.quick = quick
+        self.icon_w = icon_w
+        self.icon_h = icon_h
         self._cancel = threading.Event()
 
     def cancel(self):
@@ -1510,7 +1537,9 @@ class StoreCoverLoader(QObject):
                 except Exception:  # noqa: BLE001
                     path = None
                 if path is not None:
-                    self.loaded.emit(self.gen, row, str(path))
+                    img = QImage(str(path))
+                    if not img.isNull():
+                        self.loaded.emit(self.gen, row, _fit_image(img, self.icon_w, self.icon_h))
         finally:
             pool.shutdown(wait=True, cancel_futures=True)
         self.finished.emit()
@@ -2336,7 +2365,7 @@ class StorePage(QWidget):
         if self._closing or not tasks:
             return
         thread = QThread(self)
-        loader = StoreCoverLoader(tasks, gen, quick=True)
+        loader = StoreCoverLoader(tasks, gen, quick=True, icon_w=self.ICON_W, icon_h=self.ICON_H)
         loader.moveToThread(thread)
         thread.started.connect(loader.run)
         loader.loaded.connect(self._on_cover_loaded)
@@ -2346,12 +2375,12 @@ class StorePage(QWidget):
         self._loaders.append(loader)
         thread.start()
 
-    def _on_cover_loaded(self, gen, row, path):
+    def _on_cover_loaded(self, gen, row, image):
         if gen != self._generation:
             return
         item = self.grid.item(row)
         if item is not None:
-            item.setIcon(QIcon(_fit_pixmap(QPixmap(path), self.ICON_W, self.ICON_H)))
+            item.setIcon(QIcon(QPixmap.fromImage(image)))
             it = item.data(Qt.UserRole)
             shown = getattr(self.details, "_item", None)
             if it is not None and shown is not None and it.get("path") == shown.get("path"):