I raised this on Solus Linux forum first, and can replicate it on this forum as well.

When I click on Choose Tags straight from the new post form, the window with blue overlay opens, but the window disappears and only the blue overlay stays.
The Choose Tags window opens normally when submitting the post with the Post Discussion button.

I have Firefox 72, but unlike the other person, it happens with maximized window as well.

    What operating system are you on? And can you try disabling any popup & ad blocking addons.

      Kylo MathieuM Just tested on Windows 10, Firefox 72.0.2 and 73.0, I was unable to duplicate, this bug has been around for quite some time but so far no one has been able to track where its coming from. The issue is a 100% guarantee on IE11 and the only fix I found for it on my own Forum was to add polyfill.io to my forums. There are two open tickets regarding this issue... flarum/core1504 and flarum/core1813

          luceos Solus Linux distribution, and just tested fresh Firefox profile without any extensions and with pop-up blocking turned off. Same result.

            I'm on windows 10 and Firefox 72.0.2 and do not experience this issue, even though I did experience it before with Firefox on MacOS 🤦

            Edit; do you have any unread notifications in Flarum?

              luceos just tried with and without notification, same result.

                luceos Not right now. But the window is still there, just invisible. Just checked with Dev tools and even the mouse cursor is changing on hover.
                Looked closer, and found out that the .fade class has set opacity attribute/property (not sure with the term) to 0 (zero). When unchecked the window appears again.

                  8 days later

                  I think this issue is not specific to new post form. When i activate an extension, same thing happens as soon as settings modal pops up. I'm also encountering with this issue seldomly on my Diff extension when i try to open the modal which shows the changes in a post. It should be a modal related issue.

                    12 days later

                    Hello,
                    Same for me with beta 12 and firefox for mobiles.
                    But it works with discuss.flarum.org.
                    Thanks

                      14 days later

                      Hi, there.
                      I also encountered the same problem with flarum-beta-12 on my Firefox 74.0. when choosing the tag, the edit box disappeared.
                      I found a way to avoid this problem, as long as I reduce the browser's display ratio (Ctrl and -) to about 70% before selecting the tab, and then click the select tab button, the tab selection box can be opened normally.
                      Hope this information is helpful for positioning issues.
                      Kylo rastaferraille @luceos

                          2 months later