diff --git a/docs/libcurl/libcurl-env-dbg.md b/docs/libcurl/libcurl-env-dbg.md
index 471533625f..60c887bfd5 100644
--- a/docs/libcurl/libcurl-env-dbg.md
+++ b/docs/libcurl/libcurl-env-dbg.md
@@ -147,3 +147,8 @@ Make a blocking, graceful shutdown of all remaining connections when
 a multi handle is destroyed. This implicitly triggers for easy handles
 that are run via easy_perform. The value of the environment variable
 gives the shutdown timeout in milliseconds.
+
+## `CURL_H2_STREAM_WIN_MAX`
+
+Set to a positive 32-bit number to override the HTTP/2 stream window's
+default of 10MB. Used in testing to verify correct window update handling.
diff --git a/lib/http2.c b/lib/http2.c
index 88fbcceb71..a1221dcc51 100644
--- a/lib/http2.c
+++ b/lib/http2.c
@@ -44,6 +44,7 @@
 #include "connect.h"
 #include "rand.h"
 #include "strdup.h"
+#include "strparse.h"
 #include "transfer.h"
 #include "dynbuf.h"
 #include "headers.h"
@@ -141,6 +142,9 @@ struct cf_h2_ctx {
   uint32_t goaway_error;        /* goaway error code from server */
   int32_t remote_max_sid;       /* max id processed by server */
   int32_t local_max_sid;        /* max id processed by us */
+#ifdef DEBUGBUILD
+  int32_t stream_win_max;       /* max h2 stream window size */
+#endif
   BIT(initialized);
   BIT(via_h1_upgrade);
   BIT(conn_closed);
@@ -166,6 +170,18 @@ static void cf_h2_ctx_init(struct cf_h2_ctx *ctx, bool via_h1_upgrade)
   Curl_hash_offt_init(&ctx->streams, 63, h2_stream_hash_free);
   ctx->remote_max_sid = 2147483647;
   ctx->via_h1_upgrade = via_h1_upgrade;
+#ifdef DEBUGBUILD
+  {
+    const char *p = getenv("CURL_H2_STREAM_WIN_MAX");
+
+    ctx->stream_win_max = H2_STREAM_WINDOW_SIZE_MAX;
+    if(p) {
+      curl_off_t l;
+      if(!Curl_str_number(&p, &l, INT_MAX))
+        ctx->stream_win_max = (int32_t)l;
+    }
+  }
+#endif
   ctx->initialized = TRUE;
 }
 
@@ -285,7 +301,15 @@ static int32_t cf_h2_get_desired_local_win(struct Curl_cfilter *cf,
      * This gets less precise the higher the latency. */
     return (int32_t)data->set.max_recv_speed;
   }
+#ifdef DEBUGBUILD
+  else {
+    struct cf_h2_ctx *ctx = cf->ctx;
+    CURL_TRC_CF(data, cf, "stream_win_max=%d", ctx->stream_win_max);
+    return ctx->stream_win_max;
+  }
+#else
   return H2_STREAM_WINDOW_SIZE_MAX;
+#endif
 }
 
 static CURLcode cf_h2_update_local_win(struct Curl_cfilter *cf,
@@ -302,6 +326,13 @@ static CURLcode cf_h2_update_local_win(struct Curl_cfilter *cf,
     int32_t wsize = nghttp2_session_get_stream_effective_local_window_size(
                       ctx->h2, stream->id);
     if(dwsize > wsize) {
+      rv = nghttp2_session_set_local_window_size(ctx->h2, NGHTTP2_FLAG_NONE,
+                                                 stream->id, dwsize);
+      if(rv) {
+        failf(data, "[%d] nghttp2 set_local_window_size(%d) failed: "
+              "%s(%d)", stream->id, dwsize, nghttp2_strerror(rv), rv);
+        return CURLE_HTTP2;
+      }
       rv = nghttp2_submit_window_update(ctx->h2, NGHTTP2_FLAG_NONE,
                                         stream->id, dwsize - wsize);
       if(rv) {
diff --git a/tests/http/test_02_download.py b/tests/http/test_02_download.py
index 4b9ae3caef..b55f022338 100644
--- a/tests/http/test_02_download.py
+++ b/tests/http/test_02_download.py
@@ -313,9 +313,9 @@ class TestDownload:
         assert httpd.stop()
         assert httpd.start()
 
-    # download via lib client, 1 at a time, pause/resume at different offsets
+    # download serial via lib client, pause/resume at different offsets
     @pytest.mark.parametrize("pause_offset", [0, 10*1024, 100*1023, 640000])
-    @pytest.mark.parametrize("proto", ['http/1.1', 'h2', 'h3'])
+    @pytest.mark.parametrize("proto", ['http/1.1', 'h3'])
     def test_02_21_lib_serial(self, env: Env, httpd, nghttpx, proto, pause_offset):
         if proto == 'h3' and not env.have_h3():
             pytest.skip("h3 not supported")
@@ -332,6 +332,29 @@ class TestDownload:
         srcfile = os.path.join(httpd.docs_dir, docname)
         self.check_downloads(client, srcfile, count)
 
+    # h2 download parallel via lib client, pause/resume at different offsets
+    # debug-override stream window size to reproduce #16955
+    @pytest.mark.parametrize("pause_offset", [0, 10*1024, 100*1023, 640000])
+    @pytest.mark.parametrize("swin_max", [0, 10*1024])
+    def test_02_21_h2_lib_serial(self, env: Env, httpd, pause_offset, swin_max):
+        proto = 'h2'
+        count = 2
+        docname = 'data-10m'
+        url = f'https://localhost:{env.https_port}/{docname}'
+        run_env = os.environ.copy()
+        run_env['CURL_DEBUG'] = 'multi,http/2'
+        if swin_max > 0:
+            run_env['CURL_H2_STREAM_WIN_MAX'] = f'{swin_max}'
+        client = LocalClient(name='hx-download', env=env, run_env=run_env)
+        if not client.exists():
+            pytest.skip(f'example client not built: {client.name}')
+        r = client.run(args=[
+             '-n', f'{count}', '-P', f'{pause_offset}', '-V', proto, url
+        ])
+        r.check_exit_code(0)
+        srcfile = os.path.join(httpd.docs_dir, docname)
+        self.check_downloads(client, srcfile, count)
+
     # download via lib client, several at a time, pause/resume
     @pytest.mark.parametrize("pause_offset", [100*1023])
     @pytest.mark.parametrize("proto", ['http/1.1', 'h2', 'h3'])