The title of this diff is not quite right, it's not just merely reading from thumbs.db it's requesting the OS for a thumbnail or icon for a file, if no cached thumbnail is found, the file is still loaded (by the OS in a separate process) and a thumbnail extracted, the same work is still being done, just by someone else, i'm not convinced having this additional codepath is worth it, if dealing with thumbnails is slow, we ought to improve that in a way that benefits all platforms.

Since the thumbnails of a file are generated and cached when requested by Blender or not, for me, using this cache would still be an advantage.

If not cached, Blender requests the thumbnail and then OS creates and caches it in a way that can be used by other file explorers.

I'm not sure if the work the OS does to create thumbnails is equivalent to Blender, for OneDrive for example we can see the thumbnails even though the file is not loaded.

Another approach would be to use IThumbnailCache::GetThumbnail, but called with WTS_INCACHEONLY. This can be called from a UI thread since this returns immediately with an image if it is already in the cache, but does not make one if not.

Then this could be a tool as part of the thumbnail extraction chain rather than replace it. So used first, or only in the case of offline files. Would certainly come in handy when the user has software installed that provides a custom thumbnail provider. And would also get around our current self-imposed limitation of not thumbnailing images that have file size greater than 100MB. If the OS has already thumbnailed it - whatever it is - just show it.

HBITMAP GetThumbnailFromCache(const WCHAR *path, const size_t width, const size_t height)
{
  HRESULT hr = CoInitialize(nullptr);

  IShellItem *item = nullptr;
  hr = SHCreateItemFromParsingName(path, nullptr, IID_PPV_ARGS(&item));

  IThumbnailCache *cache = nullptr;
  hr = CoCreateInstance(CLSID_LocalThumbnailCache, nullptr, CLSCTX_INPROC, IID_PPV_ARGS(&cache));

  ISharedBitmap *shared_bitmap;
  hr = cache->GetThumbnail(
      item, width * height, WTS_INCACHEONLY, &shared_bitmap, nullptr, nullptr);

  HBITMAP hbitmap = NULL;
  hr = shared_bitmap->GetSharedBitmap(&hbitmap);

  CoUninitialize();

  return hbitmap;
}