Which ImageIO thumbnail options matter, and why does a thumbnail come out sideways or at the wrong size?

Predict first

What sizes do the thumbnails have?

import Foundation
import ImageIO
import CoreGraphics
import UniformTypeIdentifiers

// A 400x300 test image, stored as a JPEG whose EXIF orientation says "rotate 90° clockwise" (6).
let space = CGColorSpace(name: CGColorSpace.sRGB)!
let ctx = CGContext(data: nil, width: 400, height: 300, bitsPerComponent: 8, bytesPerRow: 0, space: space,
                    bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)!
ctx.setFillColor(red: 0.2, green: 0.5, blue: 0.9, alpha: 1)
ctx.fill(CGRect(x: 0, y: 0, width: 400, height: 300))
let data = NSMutableData()
let dest = CGImageDestinationCreateWithData(data, UTType.jpeg.identifier as CFString, 1, nil)!
CGImageDestinationAddImage(dest, ctx.makeImage()!, [kCGImagePropertyOrientation: 6] as CFDictionary)
CGImageDestinationFinalize(dest)

let source = CGImageSourceCreateWithData(data, nil)!
let props = CGImageSourceCopyPropertiesAtIndex(source, 0, nil) as! [CFString: Any]
print("stored pixels:", props[kCGImagePropertyPixelWidth]!, "x", props[kCGImagePropertyPixelHeight]!,
      "| orientation tag:", props[kCGImagePropertyOrientation]!)

func thumb(_ extra: [CFString: Any]) -> CGImage {
    var o: [CFString: Any] = [kCGImageSourceCreateThumbnailFromImageAlways: true, kCGImageSourceThumbnailMaxPixelSize: 100]
    o.merge(extra) { $1 }
    return CGImageSourceCreateThumbnailAtIndex(source, 0, o as CFDictionary)!
}
let plain = thumb([:])
let upright = thumb([kCGImageSourceCreateThumbnailWithTransform: true])
print("thumbnail without transform:", plain.width, "x", plain.height)
print("thumbnail with transform:", upright.width, "x", upright.height)
let big = thumb([kCGImageSourceThumbnailMaxPixelSize: 1000])
print("max pixel size 1000 on a 400x300 source:", big.width, "x", big.height)
print("thumbnail: bytesPerRow", plain.bytesPerRow, "bitsPerPixel", plain.bitsPerPixel)

• 100 x 75 without the transform, 75 x 100 with it, and no upscaling
• 75 x 100 in both cases
• 100 x 75 in both cases, upscaled to 1000