I quite like the idea, definitely makes the thumbnails more useful. It adds helpful context and works better with cases like VSE only files.

Issue is that I see use-cases where you would want to only see the scene, not the UI. There are also users who do not load .blend files with the UI. That's why we have the "Load UI" option in the file opening operator and in the Preferences.
We could do something similar for the saving: Add an operator option for the saving, and if needed a Preferences option.

@Julian Eisel (Severin) - It adds helpful context and works better with cases like VSE only files.

I tried capturing just the contents of the largest editor. That worked in many cases, but not enough. I then tried using the largest editor only if over an amount of area, like 33% of total, and doing the entire screen otherwise. But that resulted in a bit of a mish-mash.

I also liked how this gives fairly consistent proportions to them, which made it easier to compare them - something I hadn't thought of. It is a bit jarring when they differ because your brain wants to know why they different. I couldn't think of any way of cropping the result to be more square without cutting off something important. Stretching was horrible too. An interesting thought I did not explore was to simply the display a bit before capture and restore afterward, so turning off overlays etc. But in the end I just liked it being a perfect capture.

I also like the thought of this just being different from the other thumbnails. So blend could look like this, objects could be a solid (random color perhaps) render, mesh could be wireframe, etc to make them all distinct.

Issue is that I see use-cases where you would want to only see the scene, not the UI. There are also users who do not load .blend files with the UI. That's why we have the "Load UI" option in the file opening operator and in the Preferences.

Not arguing against that, but not sure. The "Load UI" is something you select while loading; the blend file itself continues to contain the UI whether you load it or not. I guess I mean the thumbnail does show the file as it is, even if you don't load it all. Or, thought of differently, it might be the arrangement that you see in the thumbnail that causes you to deselect "Load UI". So this new thumbnail type helped in that decision.

We could do something similar for the saving: Add an operator option for the saving, and if needed a Preferences option.

Yes, that is an option. I just love the idea of ripping that code out of there for something simpler. If the preference is for a render of the scene then we are stuck with having to do things differently if no camera. And I don't like the thought of users having to choose something different just because they are working without a 3DView, like with VSE

@Julian Eisel (Severin):

Another example that I don't like quite as much is to ED_screen_state_toggle before and after capture of the largest editor:

It does show a bit more of the scene and still works most of the time for VSE, depending on the layout. But I miss seeing the layout now...

Surprisingly, thumbnails of this size that capture the entire UI could be used like this...

The above seriously just a 256x146 image blown up to full size, blended with a solid mid-gray layer.

Some notes:

• Interested to know if some users prefer the existing behavior (as already suggested, this could be a preference).
• Is there a sane default when used with multiple windows?
• The patch needs updating for master.
• Is image scaling ever a performance bottleneck? Someone users might have a blender window spread across 2x 4k monitors for e.g, I'd prefer even in less common cases that this doesn't cause a noticeable performance hit. If so, we should be able to work-around this.

Thanks @Campbell Barton (campbellbarton)! I could tell that some of the naming here was a bit dodgy and needed some help. Probably needed more coffee.

Note that changing the default is still waiting on feedback from artists in the blender-institute studio as discussed last meeting.

In D10492#283568, @Campbell Barton (campbellbarton) wrote:

Note that changing the default is still waiting on feedback from artists in the blender-institute studio as discussed last meeting.

Yes, absolutely. It is far more important that actual users think it useful.