Description:
after merging cells the border style doesn't show as expected. Parts of the border line disapear randomly.
Checked on another computer and it happens as well.
If the window is minimized and show again, the borders show correctly, but, as soon as the spreadsheet is rolled up or down, or changed to pagebreak mode, some parts of the border disappear.
It happens on normal but not on preview mode. It prints as expected.


Steps to Reproduce:
1) apply a border style to a group of cells
2) merge the cells
3) borders show or not randomly
4) apply a new border style to the merged cells
5) sections of the border don't show
6) minimize window and then show it again
7) borders show as expected
8) roll rows up or down
9) sections of the borders disappear

Actual Results:  
parts of the border don't show on screen

Expected Results:
the border style applied should not change or show partially


Reproducible: Always


User Profile Reset: Yes



Additional Info:
Did not happen on Libreoffice 5.4


User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36