What is preventing me from adding a button to a button group in the Ribbon Designer?

Question

The Problem

I'm trying to add a new button to a button group using the Visual Ribbon Designer for a Word VSTO Add-In in Visual Studio 2017. When I drag a button over a button group, I normally see options for placement represented by black lines. However, when I drag the new button over a particular button group, I get the 'unavailable' cursor, as seen in the first screenshot below.

I have yet to figure out what's preventing me from adding a new button.

Screenshots

1.

2.

Research and Attempts

The button group in question is not Locked.
There doesn't seem to be a maximum to the number of buttons I can add to a button group.
If I add a new group or button group, I am able to add new buttons as usual.
If I try the 'click the ribbon control, then click where you want it' method of adding a control, I can add a button to the button group, but only to the beginning or end (screenshot 2).
I can add the button control wherever I want if I edit Ribbon.Designer.cs manually, though I'm loath to do have to always do that.

Video!

Ah! Thanks. I'd forgotten to mention that I could do that, but it doesn't work in the way you'd want or expect. I'll update the question. — ebwb, Dec 11 '18 at 18:07
then can you move it by code in `Ribbon.Designer.cs` ? (change the add order.) — Malick, Dec 11 '18 at 18:17
Yes - I'm able to place it wherever I want by writing the normally auto-generated code for it, and that's what I've been doing so far. — ebwb, Dec 11 '18 at 18:22
ok, I think I had a similar problem once, I have edited the auto generated code to solve it. Let's see if someone has a proper answer and explanation. — Malick, Dec 11 '18 at 18:31
do you take the button from the "ribbon office" group in the tool panel ? — Malick, Dec 11 '18 at 18:32
Hmm.. In my toolbox, it's called "Office Ribbon Controls". Is that the same thing? It's one of the toolbox sections that automatically shows up when I select the `Ribbon.cs [Design]` tab. — ebwb, Dec 11 '18 at 18:36
yes this is the section I'm talking. Sorry I have no other ideas. — Malick, Dec 11 '18 at 18:37
I can't recall ever having seen this before, or mention of it. I have to wonder whether something has been "corrupted"... If you delete the entire group (the code behind will remain intact!) then recreate it (and just link the buttons to the existing code)? — Cindy Meister, Dec 11 '18 at 19:33
If I create a new group (either a new group next to the offending group, or a new group to replace the offending group), I'm able to add buttons. Others on this project don't have this issue, though, and the problem remains even if I pull down a fresh copy of the solution from our version control. — ebwb, Dec 11 '18 at 20:32

score 0 · Accepted Answer · answered Dec 18 '18 at 23:44

I still don't have an answer, but I do have a workaround!

If I hold the left mouse button while dragging the button toward the offending group, and I click and release the right-click button while hovering over that group, the glyph appears indicating that it is legal to drop the button in that spot.