idk what caused it, it started happening to me a couple of days ago, I finally managed to fix it by:

1) Closing Unity
2) Back up the "Library" folder inside your project
3) Delete it
4) Load your project and wait for it to generate the folder again
4.2) It'll show a lot of nullException errors in the console and the scene and game windows will apear empty, don't worry
5) Close Unity
6) Paste the backed up Library folder inside your project and replace everything when it asks
7) Load your project again, and everything will be fixed, no errors, all the windows normal again, and the inspector has a scrool bar again!, everything is fixed.

Hope this helps someone that sees this in the future, I had been very annoyed with this bug.

Thanks Mookana, that works. You can also wrap around from the top to bottom this way. You can also type the first letter multiple times to navigate, only works well if the first letter isn't super common.

This needs to be fixed..

ok bois, i figured out a small workaround that might help some of you while waiting for this to get fixed. when trying to scroll down a large enum dropdown, mouseover an item and then hold the DOWN/UP arrow on your keyboard to do a rapid scroll. press enter to choose the item.

Unhappy to report, still there in the 2020.1.7f1 version.

This issue is present in Unity 2019.3, five years later! It's very annoying.

This is annoying that this has existed since 4.3. There is no scroll, and what's more, if you have a large list you have to click down 200 times. It doesn't remember the previous position or start on where the current option is. It doesn't allow you to display it in multiple rows. It's completely useless.

Workaround: use an autoclicker, either for the arrow keys, or for mouse clicks while cursor is on the arrow icon in the dropdown.

Unity wont allow us to dock windows from code -> I wasted time implementing dockable GUIs that can be rendered indifferently from within A)an Inspector, B)an editor window, C)a property Drawer (you name it) without needing a hack, then i was certain i was finished with what i thought was an exception in the ocean of good quality unity principles. Until i wanted to be able to display an acceptable enum popup that can display more than one entry without destroying the CPU or filling you screen with a slow ass popup.

I thought i would work around this by making a custom enum property drawer for enums, and it worked perfectly well -> insanely fast, insanely practical, but most of all, can be coded by any monkey on the planet. But, hey, here is Unity for you, the pretty as hell dropdown list that takes zero CPU appear behind the other inspector controls, GUI depth wont make a dam difference.

Unity is supposed to make us gain time, not the opposite.

This is definitely slowing down our workflow quite a bit with our big project. We'd be really happy if this was finally fixed.

Still present in U5.4.1f1