How will you combine multiple "keystrokes"?

Commands should have a shouldCombine method to determine if two commands should be combined. If true, the method combine should be used to add a command to the last command on the stack. shouldCombine can filter by word boundary, fox example, to make commands that undo per-word.

When a character is typed, look through the stack for other single characters, then combine them into a single command.

Fill a queue with the keystrokes until a blank character is reacged

The stack stores each word depending on when a space was inserted. It would have to take into account the first word (which has no space before it) and the last word (which has no space after it).

Every time a space occurs, every time another command is used, or after a certain timeout.

CRLF or \n as well.

Use space as a delimiter or in code, you could use the { } respectively to undo entire lines.

It will undo until it finds a space character

a legacy is a religious path