Serving Polymer PWA with nginx reverse proxy - nginx

Question

I'm trying to serve my Polymer PWA with an HTTP/2 reverse proxy using nginx, but I cannot get it to work properly. The PWA is served unbundled with prpl-server at 127.0.0.1:38765, which works fine. My prpl-server looks like this:
const express = require('express')
const prpl = require('prpl-server')
const config = require('./build/polymer.json')
const app = express()
const port = 38765
app.get('*', prpl.makeHandler('./build/', config))
app.listen(port)
and my nginx config at /etc/nginx/sites-available/default looks like this:
upstream app {
server 127.0.0.1:38765;
keepalive 64;
}
server {
listen 443 ssl http2 default_server;
listen [::]:443 ssl http2 default_server;
server_name app; # or full domain? tried both, doesn't work
location / {
proxy_pass http://app$request_uri;
proxy_redirect off;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;
proxy_set_header X-Forwarded-Proto-Version $http2;
proxy_set_header Host $http_host;
proxy_set_header X-NginX-Proxy true;
proxy_set_header Connection "";
# Cache Controls
# This section sets response expiration which prevents 304 not modified
expires 0;
add_header Pragma public;
add_header Cache-Control "public";
access_log off;
# Security Patches
# This section are security patches in case the client overrides
# these values, the server re-enables it and enforce its rules
add_header X-XSS-Protection "1; mode=block";
add_header X-Frame-Options "deny";
add_header X-Content-Type-Options "nosniff";
}
ssl on;
ssl_session_cache shared:SSL:5m;
ssl_session_timeout 1h;
ssl_certificate /etc/nginx/ssl/cert.pem;
ssl_certificate_key /etc/nginx/ssl/privkey.pem;
ssl_dhparam /etc/nginx/ssl/dhparam.pem;
}
When I go to the page, all dependencies seem to be downloaded over h2 except for ma-app.html (the app shell), which gives me a 502 error. All other files download with a 200 status and have the same size (minus some compression) as when I go to port 38765 directly, but the page is blank.
Am I missing something? Why doesn't the shell download properly? All files' request URLs are exactly the same for the nginx reverse proxy as for the prpl-server except for the port number.
Screenshots
It works when I access the prpl-server directly:
Does not work when I go through the nginx reverse proxy:
Some info for the failed request:

Answer 1

The problem had something to do with the buffer size being too small, as mentioned here: https://github.com/Polymer/prpl-server-node/issues/50#issuecomment-333270848.
I added
proxy_buffer_size 128k;
proxy_buffers 32 32k;
proxy_busy_buffers_size 128k;
in the location section of the nginx config and now the thing works.