A Scriptable script that creates "invisible" widget backgrounds based on your iOS wallpaper, and then either uses them as a Scriptable widget background or exports to your camera roll.

mz_invisible_widget.js

// This widget was created by Max Zeryck @mzeryck

/*
You can't add commit messages to gists, so I just want to say thanks to everyone who has used, modified, 
and enjoyed this script! This version adds the ability to export to the Files app thanks to Simon (@simonbs), 
and Plus-sized phones thanks to /u/Alekhat-akatsuki. I hope to add support for the iPhone 12 series as soon as 
I can get my hands on screenshots from the new phones. If you'd like to help, I just need to see 3 small widgets 
in a diagonal pattern, like this: https://imgur.com/a/rRzLrG2 It doesn't matter which widgets, as long as they 
contrast with the background.
*/

// Widgets are unique based on the name of the script.
const filename = Script.name() + ".jpg"
const files = FileManager.local()
const path = files.joinPath(files.documentsDirectory(), filename)

if (config.runsInWidget) {
  let widget = new ListWidget()
  widget.backgroundImage = files.readImage(path)
  
  // You can your own code here to add additional items to the "invisible" background of the widget.
  
  Script.setWidget(widget)
  Script.complete()

/*
 * The code below this comment is used to set up the invisible widget.
 * ===================================================================
 */
} else {
  
  // Determine if user has taken the screenshot.
  var message
  message = "Before you start, go to your home screen and enter wiggle mode. Scroll to the empty page on the far right and take a screenshot."
  let exitOptions = ["Continue","Exit to Take Screenshot"]
  let shouldExit = await generateAlert(message,exitOptions)
  if (shouldExit) return
  
  // Get screenshot and determine phone size.
  let img = await Photos.fromLibrary()
  let height = img.size.height
  let phone = phoneSizes()[height]
  if (!phone) {
    message = "It looks like you selected an image that isn't an iPhone screenshot, or your iPhone is not supported. Try again with a different image."
    await generateAlert(message,["OK"])
    return
  }
  
  // Prompt for widget size and position.
  message = "What size of widget are you creating?"
  let sizes = ["Small","Medium","Large"]
  let size = await generateAlert(message,sizes)
  let widgetSize = sizes[size]
  
  message = "What position will it be in?"
  message += (height == 1136 ? " (Note that your device only supports two rows of widgets, so the middle and bottom options are the same.)" : "")
  
  // Determine image crop based on phone size.
  let crop = { w: "", h: "", x: "", y: "" }
  if (widgetSize == "Small") {
    crop.w = phone.small
    crop.h = phone.small
    let positions = ["Top left","Top right","Middle left","Middle right","Bottom left","Bottom right"]
    let position = await generateAlert(message,positions)
    
    // Convert the two words into two keys for the phone size dictionary.
    let keys = positions[position].toLowerCase().split(' ')
    crop.y = phone[keys[0]]
    crop.x = phone[keys[1]]
    
  } else if (widgetSize == "Medium") {
    crop.w = phone.medium
    crop.h = phone.small
    
    // Medium and large widgets have a fixed x-value.
    crop.x = phone.left
    let positions = ["Top","Middle","Bottom"]
    let position = await generateAlert(message,positions)
    let key = positions[position].toLowerCase()
    crop.y = phone[key]
    
  } else if(widgetSize == "Large") {
    crop.w = phone.medium
    crop.h = phone.large
    crop.x = phone.left
    let positions = ["Top","Bottom"]
    let position = await generateAlert(message,positions)
    
    // Large widgets at the bottom have the "middle" y-value.
    crop.y = position ? phone.middle : phone.top
  }
  
  // Crop image and finalize the widget.
  let imgCrop = cropImage(img, new Rect(crop.x,crop.y,crop.w,crop.h))
  
  message = "Your widget background is ready. Would you like to use it as this script's background, or export the image for use in a different script or another widget app?"
  const exportPhotoOptions = ["Use for this script","Export to Photos","Export to Files"]
  const exportPhoto = await generateAlert(message,exportPhotoOptions)
  
  if (exportPhoto == 0) {
    files.writeImage(path,imgCrop)
  } else if (exportPhoto == 1) {
    Photos.save(imgCrop)
  } else if (exportPhoto == 2) {
    await DocumentPicker.exportImage(imgCrop)
  }
  
  Script.complete()
}

// Generate an alert with the provided array of options.
async function generateAlert(message,options) {
  
  let alert = new Alert()
  alert.message = message
  
  for (const option of options) {
    alert.addAction(option)
  }
  
  let response = await alert.presentAlert()
  return response
}

// Crop an image into the specified rect.
function cropImage(img,rect) {
   
  let draw = new DrawContext()
  draw.size = new Size(rect.width, rect.height)
  
  draw.drawImageAtPoint(img,new Point(-rect.x, -rect.y))  
  return draw.getImage()
}

// Pixel sizes and positions for widgets on all supported phones.
function phoneSizes() {
  let phones = {  
  
    // 11 Pro Max, XS Max
    "2688": {
      small:  507,
      medium: 1080,
      large:  1137,
      left:  81,
      right: 654,
      top:    228,
      middle: 858,
      bottom: 1488
    },
  
    // 11, XR
    "1792": {
      small:  338,
      medium: 720,
      large:  758,
      left:  54,
      right: 436,
      top:    160,
      middle: 580,
      bottom: 1000
    },
    
    
    // 11 Pro, XS, X
    "2436": {
      small:  465,
      medium: 987,
      large:  1035,
      left:  69,
      right: 591,
      top:    213,
      middle: 783,
      bottom: 1353
    },
  
    // Plus phones
    "2208": {
      small:  471,
      medium: 1044,
      large:  1071,
      left:  99,
      right: 672,
      top:    114,
      middle: 696,
      bottom: 1278
    },
    
    // SE2 and 6/6S/7/8
    "1334": {
      small:  296,
      medium: 642,
      large:  648,
      left:  54,
      right: 400,
      top:    60,
      middle: 412,
      bottom: 764
    },
    
    
    // SE1
    "1136": {
      small:  282,
      medium: 584,
      large:  622,
      left: 30,
      right: 332,
      top:  59,
      middle: 399,
      bottom: 399
    },
    
    // 11 and XR in Display Zoom mode
    "1624": {
      small: 310,
      medium: 658,
      large: 690,
      left: 46,
      right: 394,
      top: 142,
      middle: 522,
      bottom: 902 
    },
    
    // Plus in Display Zoom mode
    "2001" : {
      small: 444,
      medium: 963,
      large: 972,
      left: 81,
      right: 600,
      top: 90,
      middle: 618,
      bottom: 1146
    },
  }
  return phones
}