There is vertical line on Experimental Contextual Group Menu.
Steps to Reproduce:
0. Reset your profile using Restart in Safe Mode option then select Reset to Factory Settings. (It is not necessary.)
1. Open a New Calc document.
2. Enable Experimental Features by Tool - Options - LibreOfficeDev - Advanced then restart LibreOffice.
3. Open Calc again then choose View - User Interface - Contextual Group
4. Close Calc and open again.
There is a vertical line on the menu.
The line must be the end of menu.
User Profile Reset: Yes
I have seen the same bug on Windows 10 version of LibreOffice.
Created attachment 146087 [details]
There is a vertical line
What is more, there are another two bugs that I think they are relevant.
1) Enable Experimental Features
2) Open Writer then choose View - User Interface - Contextual Groups
3) Click the Chart (or Formula) button on the Interface and add a Chart/Formula then return to the document.
You can see a double vertical line as the picture.
I have encountered this bug on Windows and Linux Ubuntu 18.04 LTS.
Confirming the problem. When you resize the NB this line disappears so it#s a question of proper invalidating the canvas.