Comment 9 for bug 1971112

I also have this issue and have tried to troubleshoot it.

Maybe this is an interesting contribution.

If you use dconf-editor and find the "window-size" option for filechooser, you can see the dimensions of the current filechooser window. Here, you can manually set it to something reasonable (e.g., 800x600) or the default. Then press save.

https://i.postimg.cc/zvGTpYmy/filechooer.jpg

Ok, how here is the interesting bit. When you next call the filechooser from an app showing this annoying behaviour, you will see that you need to reload the deconf-editor window because changes have been made. So the app call changed the user-set size and increased it. But also changes it from default to another size (bigger).

In my desperation I even tried to make the dconf file read only and even tried to change its owner to prevent my user from writing (changing it). None of these work.

Another addition to this bug.
The behaviour where the dialogue window is not responsive is because the background window (the browser, for example) is still in focus. This is why you cannot press escape to close the filechooser. In fact, you can press ctr+s multiple times and it just opens a new instance of filechooser with a larger window than before until maximised.

I can confirm that this behaviour is shown in Brave browser (chrome), Firefox, and Telegram.