With Attachment 9227639 [details] [diff], I checked how problem happened. [LocaleService::SetAvailableLocales()](https://searchfox.org/mozilla-central/rev/b172dd415c475e8b2899560e6005b3a953bead2a/intl/locale/LocaleService.cpp#614) was called after first window was created. nsWindows::mIsRTL is initialized in nsWindow::Create(), then nsWindows::mIsRTL could not be initialized correctly.
Bug 1715990 Comment 9 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
With Attachment 9227639 [details] [diff], I checked how problem happened. [LocaleService::SetAvailableLocales()](https://searchfox.org/mozilla-central/rev/b172dd415c475e8b2899560e6005b3a953bead2a/intl/locale/LocaleService.cpp#614) was called after first window was created. nsWindows::mIsRTL is already initialized in nsWindow::Create(), then nsWindows::mIsRTL could not be initialized correctly.