Requested feature overview description.
Default to a new "Dynamic" fit setting that will attempt to best fit the post to the screen / window.

If possible, it should adjust based on screen size AND post size:
Initially, fit the post to the smallest screen dimension, if the other dimension of the post is still larger than its respective screen size, readjust to that.

The intent is simple: If viewed on a widescreen monitor, fit vertically but if the post is still too wide for the screen, fit horizontally.
If viewed in portait mode on a phone, fit horizontally but if it's still too tall, readjust to fit vertically.

The reason for the double check is to better fit portait posts on typical widescreen monitors and landscape posts on phones held in portait mode.

Why would it be useful?
Currently all posts default to a horizontal fit which works most of the time untill there's a post that is mostly vertical, those will be mostly visible on mobile when viewed in portrait mode, not so good on a widescreen monitor.
With this change all posts will be fully visible on screen regardless of their size and screen / window viewed in.

What part(s) of the site page(s) are affected?
The main affected part would be Posts.

In your account settings, you can set how posts initially appear when you open up their page: 850 pixels, fit to the screen, fit to the height of the screen, and the default original size. Is this what you are looking for?

clawstripe said:
In your account settings, you can set how posts initially appear when you open up their page: 850 pixels, fit to the screen, fit to the height of the screen, and the default original size. Is this what you are looking for?

That just lets me change the default.
I change a LOT between devices and 850 is bad on all my devices,
scale to fit is just horizontal fit (the large posts that are vertical are mostly off screen)
vertical fit works well enough on PC but when moving to mobile, all wide posts show off.