Steps:
Create a new category, click Save Category then Back to category
Click New Topic
Paste a link in the topic’s title field
Nothing happens even if you wait. :
Cancel and refresh the page
Click New Topic
Paste a link in the topic’s title field
Fields are automatically and properly filled:
I can replicate this. (863262a5de)
I think that you just need to refresh after creating a category. I don’t think that the category list is in tree message bus since it rarely changes.
I’m not sure why my topic was moved from #bug to #support; I’m reporting a (very minor) faulty behavior, not asking for a solution (I’m aware that the solution is refreshing the page) 
Yeah. If I’m not mistaken, There was kind of a similar issue on the user preferences, where the page wasn’t fully refreshing when changing the theme. Then the logo didn’t change as it should have according to the theme change, and it was modified so clicking “Save” after modifying the preferences does a full reload (at least for a theme change). Am I wrong?
But even so, I acknowledge that my report is a very, very minor inconvenience and an edge case and I can see it not being fixed ever. Thought it was worth a report though since this kind of issue can potentially happen in other areas… 
Hmm. I don’t know.
Inconveniences are not bugs.
Categories are changed less frequently than settings. I doubt this qualifies as a bug, but you can move it back (I wasn’t the one who moved it) if you’d like a member of team to decide.
Oh, I (we) don’t care that much, this report can slowly falls into the archives of indifference whatever the category it’s in 
Just to add, this has caused some confusion again recently:
