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Problem

Mac OS X has a method for inserting accented characters that is triggered by holding a key and then selecting the desired character from a contextual menu. When using this method in OS X 10.14 (Mojave) to insert accented characters in Oxygen XML Editor, the keyboard can no longer be used unless I restart the application.

Causes

This problem is caused by some sort of incompatibility issue with Java 8 and OS X 10.14 (Majave).

Solution

To prevent this problem, you should use an Oxygen XML Editor distribution that includes OpenJDK 12:
  1. Uninstall Oxygen XML Editor:
    1. All data stored in the installation directory will be removed, including any customizations or any other data you have stored within that directory. Therefore, you should make a backup of any data you want to keep.
    2. Remove the application by manually deleting the installation folder and all its contents.
  2. Reinstall Oxygen XML Editor using the distribution that includes OpenJDK:
    1. Go to the Oxygen Download page.
    2. In the Mac OS X tab, click on the macOS 10.11 and later (Includes OpenJDK 12.0.1) link to download this particular distribution.
    3. Follow the instructions to install the downloaded installation package.