IINA will show the thumbnail for time when you hover the progress slider.
The thumbnails will be cached for performance, and you can clear them in Preferences. This feature can also be turned off completely.
Now IINA will open videos in new windows by default. Pressing Option (⌥) when using Open (⌘O) or Open URL (⇧⌘O) to open videos in currrent window.
We added a welcome window at launch to avoid confusing new users. If you don't like it, you can switch back to "show open panel" or "do nothing" in Preferences easily.
You can also make it default to open new videos in current window in Preferences.