When editing an event, I am trying to select the category of the event. The list of available categories shows in the drop-down box and I highlight the one required, but the mouse click will not select it, and when I press the return key it seems to choose a random category and not the one I have chosen. It seems to be more luck than judgement if I eventually get the correct one. Sometimes it even chooses multiple categories instead of just one.

Reproducible: Always

Steps to Reproduce:
1. Select event to edit
2. Click on the arrow of the categories field to show the list of categories
3. Highlight the required category, then left-click the mouse (doesn't work) so press Enter key
Actual Results:  
Random category shows in the categories box, sometimes multiple categories.

Expected Results:  
Chosen category should be shown in the categories box.