Created attachment 159825 [details]
2 documents to show the problem and screenshots

When rendering with Skia is activated Writer in LO 7.0.0 builds from /daily/master/Win-x86_64@tb77-TDF/2020-04-08_04.59.52/ and newer shows artefacts in some situations when text parts are selected.
In the attached zip file there are some screenshots: ("Writer_selected_text_selected_scrolled_up_artefacts_+Options_view_Skia_is_activated.jpg" + "Writer_selected_text_selected_scrolled_up_artefacts_+About_(old)_Skia_is_activated.jpg").
 
The artefacts appear not always but on the way described below the behavior can be forced:
- open "Test_artefacts.odt" from the zip attached file
- select text as shown in screenshot "Writer_document_just_opened_text_selected_no_artefacts_+About_(new).jpg"
- scroll down the text
- --> in the moment the selected text starts rolling out of the window on the upper edge the artefacts appear
- select other parts of the text --> artefacts appear too

The artefacts disappear completely or partially when selected text is rolled out from the window first and then rolled in or completely when document is newly painted e.g. by changing the zoom factor.

The problem exists in a similar form in Calc, see screenshot "Calc_artefacts.jpg" in the zip file and document "Corona-Entwicklung_D.ods" for demonstration.

In LO 7.0.0 builds from 2020-04-07 and older the problem doesn't exist.