fix: prevent black screen when loading slides with subsections in Qt presenter

When loading a slide with subsections in --subsections=all mode,
start playing first to load the first frame, then immediately pause
at position 0. This prevents the black screen that occurs when
pausing before the first frame is loaded.

Fixes the black background issue when transitioning from slides
without subsections to slides with subsections.

Author: krystophny

manim_slides/present/player.py

@@ -429,10 +429,11 @@ def load_current_slide(self) -> None:
         else:
             self.media_player.setLoops(1)
 
-        start_paused = use_subsections and self.subsection_mode == SubsectionMode.all
-        self.load_current_media(start_paused=start_paused)
-        if use_subsections:
+        # Always start playing to load the first frame, then pause if needed
+        self.load_current_media(start_paused=False)
+        if use_subsections and self.subsection_mode == SubsectionMode.all:
             self.media_player.setPosition(0)
+            self.media_player.pause()
 
     def load_previous_slide(self) -> None:
         self.playing_reversed_slide = False