If you keep the default configuration when you install Safari 12 and try to use one of the icons to insert an image or URL link (anything that pops up an entry panel) you’ll see that Safari seems to become stuck…
This is due to the new default for “Enable/Disable Pop-up Blocker” which blocks those popups.
Here this is a good thing and we want them - so you need to allow popups for the Arduino Forum.
To do this:
try to create a new post and click on the “insert image” icon. That will fail (otherwise you are all good)
escape out of this / cmd-R to reload your page and exit from the endless loop
In the menu, go to Safari/Preferences
In the panel that opens, pick “Websites” in the tool bar.
Select “Pop-up Windows” on the left side.
you’ll see a line for “forum.arduino.cc” → in the popup to the right, select
quit Safari and relaunch. you should be good to go