Refocusing the key dropdown should jump to the correct position

[flaimo]

1. select node and click the add new key/value pair button
2. type landuse in the, already focused, key field
3. press tab to switch to the value field
4. press shift+tab to switch back to the key field
5. press cursor down

current behavior (which i consider wrong):

1. after the keypress the dropdown opens up, but nothing seems to be focused
2. press cursor down again. the very first entry in the dropdown gets selected

at this point i actually expect the following:

1. the dropdown opens up. focus is on the entry landuse which should be in the visible area of the dropdown
2. press cursor down again. the next entry after landuse should be highlighted

environment: build 4021 on mac os x

[Don-vip]

Closed as duplicate of #10470.