Link to issue number:

Fixes #19263
Closes #10545
Possible fix for #18693 and #18743

Summary of the issue:

Since #19177, if Screen Curtain fails to initialise because isScreenFullyBlack returns False, NVDA crashes (#19263).

It would appear that from time to time, bitblitting from the desktop context to the capture context in nvdaHelper/local/screenCurtain.cpp failes due to one or both of the context handles being invalid.
I have no idea why this happens, but can only surmise that it's a timing issue, as we check the validity of the contexts upon creation, so something must be invalidating them in the meantime.

I strongly suspect that this is the underlying cause of Screen Curtain failing to be activated on the Windows login screen (#18743) and after login (#18693).

Description of user facing changes:

NVDA no longer crashes if Screen Curtain fails to be enabled when initializing NVDA.

Screen curtain may be enabled more reliably at the login screen and immediately after login.

I have not included a change log entry, because #19263 was introduced as part of 2026.1 alphas, and I don't know if this PR fixes #18693 or #18743.

Description of developer facing changes:

None

Description of development approach:

• In screenCurtain._screenCurtain.ScreenCurtain.enable, add logic to automatically retry enabling the Screen Curtain a fixed number of times before admitting defeat.
  If the issue really is a race condition or other timing issue, future retries should theoretically be uneffected by the issue, so it will succeed.
  Currently the maximum number of attempts is hardcoded at 3.
• In screenCurtain._screenCurtain.ScreenCurtain.__init__, wrap self.enable in a try/except block.
• In gui.settingsDialogs.PrivacyAndSecurityDialog._ensureScreenCurtainEnableState, wrap screenCurtain.screenCurtain.enable in a try/except block, and notify the user when enabling the screen curtain has failed.

Testing strategy:

Intentionally introduced errors into screenCurtain._screenCurtain.ScreenCurtain.enable, and ensure that the error handling works correctly.

Intentionally introduced errors that only happen a fixed number of times into screenCurtain._screenCurtain.ScreenCurtain.enable, and ensure that the retry logic works correctly.

Known issues with pull request:

None

Code Review Checklist:

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