The new crossword solver has a feature called “Draft Letters,” which let you enter tentative answers that you're not ready to commit to. They're a helpful way to keep track of possible answers while you continue solving the puzzle. Draft Letters look different from regular letters (lowercase and a grey) so you can easily tell them apart.
Holding the Shift key while typing enters Draft Letters. If Caps Lock is turned on, every letter you type will be entered as a Draft Letter until Caps Lock is turned off. You can overwrite a Draft Letter by selecting the box and typing as normal, without holding shift or enabling Caps Lock.
When you use the “Check” function to validate a word, letter, or an entire puzzle, any correct Draft Letters will be automatically converted to a regular uppercase entry.