keyboard focus gets lost when starting trial mode

[robx]

On (my) firefox / macOS. Safari seems fine.

E.g., with a skyscrapers puzzle (probably not all steps required):

1. click top left
2. input '1'
3. click trial mode
4. click next cell
5. input '2' (nothing happens)
6. click outside the puzzle
7. input '2' (accepted)

In step 5, the number should already be accepted.