A dead key followed by a Space should generate just one character. For example, a dead key circumflex followed by a Space should yield just a caret. This is the standard on all platforms.

In JavaFX on Windows a dead key followed by a Space yields the expected character but it's followed by a space character. This can cause some surprises when typing into a password field.

Normally if the user presses a dead key followed by a character that can't combine with the diacritic they should see two characters e.g. dead key circumflex followed by a q should yield ^q. The space character is an exception.

BTW, this bug has been in the system for a very long time.

To reproduce:
Configure and select the US International layout.
Launch any JavaFX app that accepts text input.
Type Shift-6 followed by a space. This is a dead key circumflex followed by a space.

Expected result:
Should see a single caret

Actual result:
A caret appears followed by a space.

For extra credit type Shift-6 followed by a 'q' to verify that you see ^q. Then press Shift-6 followed by an 'e' to verify that you see a ê (e with circumflex).