Auto-size video widget to match video aspect ratio

Detects video dimensions from first frame via QVideoSink,
sets max height on the video widget to eliminate black bars.
Resets on each new video. Uses KeepAspectRatio mode.

booru_viewer/gui/preview.py

@@ -399,9 +399,9 @@ class VideoPlayer(QWidget):
         layout.setContentsMargins(0, 0, 0, 0)
         layout.setSpacing(0)
 
-        # Video surface — inherit theme background instead of black
+        # Video surface
         self._video_widget = QVideoWidget()
-        self._video_widget.setStyleSheet("background: transparent;")
+        self._video_widget.setAspectRatioMode(Qt.AspectRatioMode.KeepAspectRatio)
         layout.addWidget(self._video_widget, stretch=1)
 
         # Player
@@ -471,6 +471,11 @@ class VideoPlayer(QWidget):
         self._player.playbackStateChanged.connect(self._on_state)
         self._player.mediaStatusChanged.connect(self._on_media_status)
         self._player.errorOccurred.connect(self._on_error)
+        # Resize video widget to match video aspect ratio
+        self._video_sized = False
+        sink = self._player.videoSink()
+        if sink:
+            sink.videoFrameChanged.connect(self._on_video_size)
         self._current_file: str | None = None
         self._error_fired = False
 
@@ -478,6 +483,8 @@ class VideoPlayer(QWidget):
         self._current_file = path
         self._error_fired = False
         self._ended = False
+        self._video_sized = False
+        self._video_widget.setMaximumHeight(16777215)  # reset max height
         self._last_pos = 0
         self._player.setLoops(QMediaPlayer.Loops.Infinite)
         self._player.setSource(QUrl.fromLocalFile(path))
@@ -554,6 +561,18 @@ class VideoPlayer(QWidget):
         else:
             self._play_btn.setText("Play")
 
+    def _on_video_size(self, frame) -> None:
+        """Resize video widget to match video aspect ratio on first frame."""
+        if self._video_sized or not frame.isValid():
+            return
+        self._video_sized = True
+        vw = frame.size().width()
+        vh = frame.size().height()
+        if vw > 0 and vh > 0:
+            available_w = self._video_widget.width()
+            scaled_h = int(available_w * vh / vw)
+            self._video_widget.setMaximumHeight(scaled_h)
+
     def _on_media_status(self, status) -> None:
         pass