Hi All,
For the Outlook inserting Horizontal Line not fully displayed, it is a known issue of Outlook with build 1912. The Microsoft Dedicated Outlook teams are aware of and investigating this problem. However, they may need some more time to analysis and fix this
problem in the future release.
We do understand this problem would affect your daily work and life, so here are some temparary workarounds for this problem.
First one, this problem occured with Outlook 1912 builds, you can try to rever the Outlook version to 1911 and then see if the problem disappears. For more details, please refer to https://support.microsoft.com/en-us/help/2770432/how-to-revert-to-an-earlier-version-of-office-2013-or-office-2016-clic.
Or open the Command Prompt and run the following commands one by one
cd %programfiles%\Common Files\Microsoft Shared\ClickToRun (Press Enter)
officec2rclient.exe /update user updatetoversion=16.0.12228.20332 (Press Enter)
after that wait for the Office updates to revert back to 1911 build, thanks.
Another one if you don't want to revert back to an old version of Outlook, when you tried to insert Horizontal Line, you could try to type ------ then press Enter key as a workaround, thanks.
We will help monitor this problem and if anyone encontered this problem, please subscribe this thread and wait for the future release with some more patience, thanks.
All your understanding and patience will be highly appreicated.
Best Regards,
Oliver