transmitted getUserMedia / MediaRecorder video format larger than requested format. How to tell?

Background:

On Windows 10 I'm using getUserMedia (gUM) and MediaRecorder in Google Chrome (v71) to capture and encode a video stream.

• I'm using the constraints parameter to gUM to tell it I want a video format of 352x288.

• I'm requesting video/webm; codecs="avc1.42E01E" as the MIME type of the encoded stream (that's H.264 boxed in Matroska).

• I'm selecting a cheezy webcam built into a laptop as the video source. It's called "EasyCamera" made by DMAX-AVC. It's tempting to call it CheezyCamera.

The video stream gets generated just fine.

Problem:

The dimensions of the encoded video in the stream are 440x360 rather than my requested 352x288. This information is embedded in the recorded stream, and only visible from the consumer of that data. Use of the various APIs reveals the gUM stream, MediaRecorder, and <video> element metadata all think the dimensions are the ones I asked for.

Of course, webcam, gUM, and MediaRecorder treat the constraints parameter as suggestions, and are free to respond with something different. In this case they respond with 440x360 when I request 352x288. This system functions as designed; that is not my problem.

To clarify, the unexpected 440x360 dimensions are only visible to the consumer of the recorded stream. I hope to find a way to know the producer-side webcam, gUM, and MediaEncoder signal chain is producing a different resolution than I requested.

How does the stream consumer know the stream dimensions? They're in the 'PixelWidth' and 'PixelHeight' Matroska boxes, and they're baked in to the H.264 stream. (Oddly enough this generated resolution isn't an integral number of 16x16 macroblocks. )

I can't parse the recorded data in the browser because it's stored in opaque blobs.

When I use a different, better, webcam (a Logitech C615) my encoded video stream is the size I requested.

My question:

Is there any way in the webcam / gUM / MediaRecorder / <video> signal chain to find the actual dimensions of the encoded stream in the browser actually recording the stream? That is, can I find the signal chain's response to my requested dimensions without decoding the generated stream?

from transmitted getUserMedia / MediaRecorder video format larger than requested format. How to tell?