@toxicdragon these look amazing, and I’m truly impressed by the quality and scale of what you’ve achieved.
One concern I have is that there don’t seem to be any file names — i.e., an app that edits files usually includes the name of the file in the header bar.
@Villeroy I don’t see why the GTK UI for LibreOffice should not be updated to a newer version.
Your criticism is generally repetitive, not particularly constructive, written without much effort or depth, and motivated by personal opinion, which is a great thing, but it is not right to try forcing it upon others.
The other UIs are still available to you, and you have no requirement to use the GTK/LibAdwaita UI. Christian/toxicdragon has put a lot of effort to ensure that this does not affect the other UIs for LibreOffice, as noted by other developers in the mailing list linked by him. You can simply recommend common ways to switch to other UIs (you did suggest one, although most people will not know to use the terminal for it), and link to the other UI options so that users can see it for themselves amd choose what they like.
Your earlier message about form controls seems misinformed, since GTK and LibAdwaita have a large library of controls, which include dropdowns, radio buttons, checkboxes, (number) spinners, and more.
Regarding flat design; while it might not be objectively perfect in terms of beauty or functionality, it fits in with the GNOME design, as well current UI design trends. This allows it to be similar to other applications, which lets users understand functionality more easily; imagine how it would be for different apps to use different icons, button placements, colours, etc. — users would be very confused and pained when trying to do their work.
Also, your last one or two comments seem a little unnecessarily hostile. The Libre Office codebase is very old and large, and Christian only started looking into it 4 or 5 months ago. It would be confusing to anyone in this context, yet Christian did a lot of work in 1 or 2 months. That does not seem to be the work of someone who is lost in the codebase. Additionaly, do you speak from experience when you say GTK is unsuitable? The fact that GTK UI already exists would suggest that GTK is actually suitable enough. And that Christian could achieved so much in suh a short time also suggests that GTK is suitable (as well as the fact that Christian is quite skilled).
Edit: the work is from Dec 2021, not 2022, as I first thought, so Christian first read the code 1 year and 4–5 months ago. However, most of the work was still achieved in the first month or two.