The javascript that updates the date display when editing a date format seems to make navigating through the string tricky.
Steps to reproduce
Go to
Regional and language > Formats
Click Edit beside any of the date formats (e.g. admin/config/regional/date-time/formats/long/edit)
Tab or mouse click into the Format string field
SHIFT+right arrow to select text
On the right arrow keypress I can see the javascript progress throbber appearing and the selection highlighting disappears making it difficult to tell how much text has been selected.
Also when trying to right arrow through the string to place the cursor somewhere in the middle of the existing string, the cursor seems to get stuck in places. It doesn't seem to like spaces or punctuation and it can take 4-5 key presses to get past them.
Again the throbber appears on each left/right keypress.