Scaling in xd

SHIFT METHOD

I've used the method of holding down the shift key while ensuring the responsive setting is set to off. I've also tried to de-select the options that appear (Fix Width, Fix Height, etc) under the Auto and Manual tabs to ensure that the shift option is used in isolation. The result is that the inner icon distorts itself (flattens).

TESTING RESPONSIVE OFF STATE

Since I know re-sizing a responsive component displays faint red lines to show distance from outer to inner, I made to sure to test for this and indeed it appeared for when responsive was turned on. When responsive was turned off, the faint red lines were not present indicating that it wasn't checking for layout boundaries.

INNER SHAPE SET TO MANUAL

Another option I tried: I selected the path within the group (that's getting distorted with resizing) and made that Manual  itself. The result from this method was just moving around the inner as I held shift and scaled the corner of the surrounding/outer square.

STOP-GAP FEATURE REQUEST: EDIT IN ILLUSTRATOR

I know these things take time, but for now it would be great if in the right-click context menu, we could have an option to edit in illustrator. The startup time for illustrator on my machine, AND transferring / testing between applications is a distracting and workflow-breaking process. Live-linking to isolate the specific feature of merging paths together in illustrator would be really ideal. 

YES, I'VE CONSIDERED THE LIBRARY SYSTEM

I know that there is a library system that I assume is connected when you open illustrator; the one where you save a "folder" of assets that you can access from any adobe application. However, this assumes that illustrator is required for the workflow... when I think many of these comments make it clear that it's an unfortunate side-task that just needs to get done in order to move forward in XD. Further, I haven't adopted the library system myself due to the objective nature of my project assets, it slowing down my machine significantly (I have a 2018 macbook pro), and my repeatable use of particular app features that makes it less of the "main event" for my workflow. Isolating it to a single task I need done from illustrator (if we can't get a working path merge feature in XD) by allowing live linking and auto-closing of illustrator when finished -- would be really great.

I still love the product but this has been a huge pain. I wanted to type out my detailed experience though, so you can get the full picture and do what you need. Thanks!

Isn't that Center option executed by holding the Alt/Option in stead of Ctrl ?