Exported videos have a wide black border

[jnbspace]

I was using shotcut to assemble some video and audio from separate sources and when I exported the video file to an MP4 the resulting video had a huge black border around it.

The original data was 40 000 separate frames (1920 x 1920 resolution), which I have assembled to a source MP4 without audio by using ffmpeg.
Then I have used shotcut to assemble two such video segments and two audio segments to create an output mp4 which I create at 1920 x 1920 and 1200 x 1200. When I play back the produced MP4 it has created it as a rectangle 1920 x 1080 and has shrunk the video itself to something much smaller in the middle of that.

Source video (post ffmpeg) - https://drive.google.com/file/d/1ZhSij_dz16VsQRRyJWxuUXgxQWtcnnNM/view?usp=sharing

Exported video (post shotcut) - https://drive.google.com/file/d/1TSizCOsOnljPaIprpJxTNaytmZFEauHe/view?usp=sharing

In the past I have done the entire process in ffmpeg, but I used shotcut as I thought it would be easier than trying to remember the 18000 or so command line parameters for ffmpeg.

If I export the file as 1920 x 1080 it fills the frame but how do I persuade shotcut to do this with a square export?

In the export phase I have explicitly set the resolution to 1920 x 1920 (or 1200 x 1200) with an aspect ratio of 16 x 16 and frame rate of 25 fps but it still mangles the output. Normally if I use shot cut I am in a 16 x 9 frame format through and it works. But how do I persuade shotcut to use a square frame?

[Hudson555x]

@jnbspace

But how do I persuade shotcut to use a square frame?

Set your Video Mode before starting your project. You still may be able to change it, but you may need to adjust filter settings after a project has been started.

Settings - Video Mode - Non-Broadcast - Square 30fps or 60fps (Presets)

Since you are exporting with 25 FPS, you can make a Custom Video Mode.
Settings - Video Mode - Custom - Add

Export settings will match Video Mode settings.
Default preset

Also make sure you don’t have an accidental filter on the Output track.
The filter symbol looks like a funnel.

[shotcut]

Shotcut includes square video modes in the Non-broadcast submenu.

Video Mode

Introduction Video Mode is the project resolution, aspect ratio, frame rate, and color space. It is important to set this as you desire at the beginning of your project. If you try to change it later Shotcut will try to retain the timing of edits, but some users have reported problems, and it is risky. Also, at this time Shotcut will not automatically adjust anything for you related to size and positioning of elements like text and picture-in-picture when changing resolution. You can think of…