mpv js extension to save the last position, dimensions of the mpv windows. i.e. open same as before. Only WINDOWS

Get-Client-Rect.ps1

Add-Type @"
using System;
using System.Runtime.InteropServices;
public class Window {
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool GetClientRect(IntPtr hWnd, out RECT lpRect);
    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool ClientToScreen(IntPtr hWnd, ref POINT lpPoint);
}
public struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;
}
public struct POINT
{
    public int x;
    public int y;
}
"@

$Handle = (Get-Process -Id $Args[0]).MainWindowHandle
$ClientRect = New-Object RECT
$Position = New-Object POINT
$Size = New-Object POINT

[Window]::GetClientRect($Handle, [ref]$ClientRect) | out-null
$Position.x = 0 # $ClientRect.Left is always 0
$Position.y = 0 # $ClientRect.Top
$Size.x = $ClientRect.Right
$Size.y = $ClientRect.Bottom
[Window]::ClientToScreen($Handle, [ref]$Position) | out-null

Write-Output "$($Position.x) $($Position.y) $($Size.x) $($Size.y)"

save_last_pos.js

// Modified by Phani Rithvij (github.com/phanirithvij)
// Extended from Pablo Bollansée's https://github.com/TheOddler/mpv-config

// Sometime the leftmost screen doesn't have id 0
// not sure yet how to detect this automatically
var leftMostScreen = 1

// Some setup used by both reading and writing
var dir = mp.utils.split_path(mp.get_script_file())[0]
var rect_path = mp.utils.join_path(dir, "last_window_rect.txt")

dump(rect_path)

// Read last window rect if present
var rect = null
try {
    rect = mp.utils.read_file(rect_path).trim().split(' ')
} catch (e) {
    dump(e)
}

if (rect != null) {

    var x = rect[0]
    var y = rect[1]
    var width = rect[2]
    var height = rect[3]
    mp.set_property("screen", leftMostScreen)
    var autofit = width + "x" + height
    mp.set_property("autofit", autofit)
    var geometry = width + "x" + height + "+" + x + "+" + y
    mp.set_property("geometry", geometry)

    // fetch the rect on load
    mp.register_event("file-loaded", fetch_rect)
}

// fetch the screen dimensions
function fetch_rect() {
    var ps1_script = mp.utils.join_path(dir, "Get-Client-Rect.ps1")
    var args = ["powershell", "-File", ps1_script, mp.utils.getpid().toString()]
    var output = mp.utils.subprocess({
            args: args,
            cancellable: false
        }).stdout
    var newRect = output.trim().split(' ')
    var fact = 1.25
    var i = 0;
    newRect.forEach(function(_) {
        newRect[i] *= fact
        newRect[i] = Math.round(newRect[i])
        i++
    })

    return newRect.join(" ")
}

// fetches and saves the dims
function fetchAndSave() {
    var output = fetch_rect()
    save_rect(output)
}

// Save the rect in a file
// Need to do this before exit to save last screen dims
function save_rect(rect) {
    mp.utils.write_file("file://" + rect_path, rect)
}

// i.e. this
mp.register_event("shutdown", fetchAndSave)

Place these under your C:\Users\<Username>\AppData\Roaming\mpv\scripts folder on Windows

Checkout my mpv-config at https://github.com/phanirithvij/mpv-config