Link to issue number:

Follow up of #14759
Fixes #14784
Partially reverts 9b97de0

Summary of the issue:

At least one custom IME is reported to no longer work in editable controls within NVDA.

Description of user facing changes

Fix the regression

Description of development approach

We no longer use the background thread to handle cancellable messages, as it seems using the background thread causes IME messages to be discareded. This means a partial revert of 9b97de0. In fact, this pr removes the background thread but keeps the event based approach to cancel calls from NVDA's watchdog.

Testing strategy:

• Ensure that Regression of #14759: Unable to use Sogou Pinyin to input Chinese characters in NVDA's editable controls #14784 can no longer be reproduced
• Ensure that nvdaHelperLocal dll import table hook for oleacc SendMessageTimeoutW not being used #14746 still can't be reproduced any more
• Ensure that NVDA also crashes and exits when an error occurs in the application it is interacting with #13335 still can't be reproduced any more

Known issues with pull request:

None known

Change log entries:

Add this pr and issue #14784 to the changelog entry for #14759

Code Review Checklist:

• Pull Request description:
  • description is up to date
  • change log entries
• Testing:
  • Unit tests
  • System (end to end) tests
  • Manual testing
• API is compatible with existing add-ons.
• Documentation:
  • User Documentation
  • Developer / Technical Documentation
  • Context sensitive help for GUI changes
• UX of all users considered:
  • Speech
  • Braille
  • Low Vision
  • Different web browsers
  • Localization in other languages / culture than English
• Security precautions taken.