Possible with Application.QueryCancelGroup event.
VBA example attached.

I have a question for a VB.NET Visio add-in:

is it possible to catch a grouping event and prevent the ungrouping conditionally, in this case I want to run some code and see if certain shapes are involved, in which case I want to stop the grouping from happening and provide the user a message about it.

I cannot find a regular Visio shapesheet setting/function where I can prevent a (locked group) shape from becoming a member of another shape group if the user performs this user action, which is why I need to do this in code.

Thank you for sharing your insights!

Good to hear that it works for you.
I don't know what the user Prompt cell is intended for. But it can store up to 64kB of text and can be used for your formulas exactly as the Value cell. So, I use it do decrease the amount of rows.
Anyhow, the prop Prompt cell is used for pop-up message when hovering the mouse over the property in the shape data window. So I wouldn't use that for other things.

I recommend to distribute your code in a central stencil that you share with your colleagues.
The documents should be stored without code as .vsdx.
That's much easier to maintain.
Have a look here for a starting point with ribbon menu:
https://visguy.com/vgforum/index.php?topic=7134.0

Use the Pencil tool.  The inflection points become visible with this tool. 

See this:  https://support.microsoft.com/en-us/office/draw-lines-and-shapes-with-the-pencil-line-arc-or-freeform-tools-9953bb53-26bf-4995-8c96-ef8674c16c15

Well, here's update to my file to bring it to a potentially meaningful closing point.  The file has 4 pages.
First page details the process for doing a manual creation.  For the example, the radii for each circle is 1/2 previous circle, and the enclosed angle is 90 degrees.  However, both the angle and radii may be any chosen value, and are independent of each other.  Directions are on the page.

2nd page is similar to first, except the process has been captured in VBA macros.  The process mimics page 1 method, but shapesheet results are different, there being 3 geometry sections as opposed to 1. 

Pages 3 and 4 look at scalability.
Page 3 shows the process of rescaling and the subsequent preservation of all shape relationships.  This means that complex spirals may be easily formed from simple structures.

Page 4 briefly looks at this complex from simple.

Hello,

MS has made such a mess of all their programs that I literally can't figure out which Visio version I have. It's part of a 365 subscription and it's the Professional edition.

I've been using Visio for about 15 years and I STILL don't know how to make the shape handles that control the curvature of a line segment between two vertices show up. For example, I drew 4 free-form line segments. I snapped the vertices at the ends together, then used union to close the shape. Great. But now I want to modify the curvature of the lines but I don't have the handles to do so. I can select and move the vertices, but I can't modify the two handles that I think each vertex has.

How do I show them?

Bonus: Is there a way to insert a vertex into a line/curve segment?

Thank you for any help, it will be greatly appreciated!

An update on my project;

The code and shapesheet formulas from Thomas worked perfectly. It just took a little bit of finagling to get it to work with the shapes I had already built. I did have to input these in for every shape, but it does what I need it to do and it's easy once you find a flow and a template. I attached samples of what I've had to build into the user.shapesync value cells.

I ended up moving the sync.refresh command to the page level to allow users to be able to click anywhere on the page and refresh. It allowed me to avoid having to add additional items to the menus of every shape.

I do wish there was a way to find how to reduce the amount of code needed for every shape, but it works and is not bogging down my build at all.

I do have a question about the user."rowname".prompt field. I don't understand how or why that works since I thought the prompt field was used to display a message to the user in some way. If other prompt fields work the same way I may be able to reduce the amount of space I'm taking up on my shape data page.

Just glad it works and that it allows me to focus on so much other stuff, like properly learning vba to program menu items, making automated color connectors and such. I'll come back with a full post showing what I'm fully trying to accomplish. Big thanks to Thomas. Yall will see another post from me pretty soon.

@RailwayKen, I wonder if and how we can be of any further help.
I've seen your posts and your explanations, but I cannot clearly see any clear requests or questions.

I've been pursuing an alternative approach, using circles and ElipitalArcTo shapesheet functions.  In theory, rather simple concept.  In practice, depending upon the "strictness" of accuracy, not that trivial to implement.  This is still a work in progress...  I present it as a teaser, should someone either be interested or want to pursue on their own.

Interestingly, it is scalable.  So smaller units, of two, or three pieces, could be cascaded to make longer, more involved spirals.  Indeed, by changing the segment size, the incremental sizing of the radii, many different types of spirals might be constructed.

The attached file uses 1/4 circular segments (90 deg) with radii decreasing buy 1/2.  If I'm not mistaken, this would be forming an Archimedes spiral. 

Crikey, options that I had never even imagined. Lots to think about. Thanks.
Linux Mint.