sowenjub

Note: You don’t need to add the new layer before doing the Geometry operation, you can just select the layer and use Geometry Add. (This works with Curve, Curves and Shape layers – amongst others – but it will, as you say, bake shapes into curves.)

What am I supposed to do with this message: "Affinity Designer removed links to broken objects in your document".
It pops up every time I open my file now, how do I get rid of it?

Obviously. The trouble is that it wasn't intentional, and the app didn't help me find about it. I'd have expected this panel to let me spot said files, or open a relevant panel.
I searched the help for "object", "link", "file" and got nothing. I finally found the "Resource Manager" which showed no particular error. I removed some files and the error went away.
I learned about that manager along the way, so that's a win!

Holding down Space Bar does it temporarily, but a View > Hide Selection Edges command would be welcomed.

Obviously. The trouble is that it wasn't intentional, and the app didn't help me find about it. I'd have expected this panel to let me spot said files, or open a relevant panel.
I searched the help for "object", "link", "file" and got nothing. I finally found the "Resource Manager" which showed no particular error. I removed some files and the error went away.
I learned about that manager along the way, so that's a win!

Do you have any linked files in your document?

@MikeTO Alright, so:
clearing things did solve it 🥳 I moved files around (changing their folder), so maybe that's how I ended up there I haven't found how to Pin files, I must be missing something obvious. I tried a right click and thought displaying as a list might reveal a checkbox of sorts.

The easiest workaround I can think of is to draw and inset it from the corners of the page half the width of your stroke. If your stroke is 30pt then inset it by 15pt. Give this box no stroke and no fill so it's invisible and send it to the back. Ensure Snap to Object Bounding Boxes is checked. Then the curve you're working with will snap perfectly inside the page diameters with no clipping.

Try mousing over the upper right corner of the thumbnail's square - does a pin icon appear? If not, click blindly where you see the pin in my screenshot below and one should appear. This must be a bug but once you've pinned one it seems like the pins show up fine from then on.

That doesn't look right. If there are no documents in your recent list it should look like the screenshot below. You're missing the left sidebar, all you have is the Account icon and it's in the wrong spot.
When you choose File > New and see the New Document dialog, are you not able to click Recent in the left sidebar and then New again to go back to the standard New Document options?
What is in your File > Open Recent menu? If there's something there, try choosing Clear Menu and then opening a document and seeing if that fixes it.

Before posting I found this post. I hoped 2 years had brought better solutions, but apparently not. I'll stick to my "duplicate, expand and hide" method for the time being. Thanks for trying to help!

I noticed that the group icon in the layers panel on Affinity Designer V2 is now a generic folder verses a small preview of the group as it was in V1.  Is this a feature that will be returning? It made navigating through various groups so much easier.  Thanks!

@Jason M
Welcome to the madhouse
In the hamburger menu for the Layers panel select...
Show Group Thumbnails

@MikeTOThanks! The pin didn't appear but clicking blindly did work. It shows up fine from then on, but only for the pinned file, not the others.

If you start to drag a marquee above a background image, it will only select the background.
You need to start to drag from outside the background image. Then it will only select the images the marquee completely surrounds., not the background image.
Alternatively, press down Ctrl + Alt and it will not select the background, only the objects the marquee surrounds. This is for a PC, I guess it is Cmd + Alt on a Mac.
Note: That will only work if the background is not selected already. 
The easiest way is to deselect the background is to click on one of the objects you want to select (which selects that instead of the background) press Ctrl + Alt and then drag around everything you want.
 

I don't know about your group but this a grouped rectangle and text frame.
 
Try dragging the bottom right handle

The frame and text scale as one.

Select the line. Go In the menu "Layer" and select "Expand Stroke". After that your line is a filled shape that you can use with compound shapes.
Hope that could help.