When you have enabled 'multi-character searches' for the a-z bar, MB will store the clicked letters for a few seconds so you are able to click another one to add it so to create a search string.
But there is no indication on the a-z bar if a letter has been clicked, and if it is still 'in memory' (i.e. if you still have time to enter the next letter before the cache gets reset again)
It would be very helpful if the clicked letters would remain highlighted (bold?, coloured?) for the duration that MB stores them before automatically resetting.