Created attachment 147275 [details]
Video capture of a small portion of the screen, showing a button+caption flashing and CPU usage graph.

In all LibreOffice applications;
With any GTK3 theme;
Also after deleting .config/libreoffice configuration folder;
Current version (previous versions were not affected, however I have not used any Libreoffice application in the past 3 months):
  Version: 6.1.3.2
  Build ID: 6.1.3-2
  CPU threads: 4; OS: Linux 4.19; UI render: default; VCL: gtk3; 
  Locale: it-IT (it_IT.UTF-8); Calc: group threaded

Issue:
Any button of the toolbars, when hovered, starts to flash the tooltip (popup caption), and flashes itself, too. Watch the video for more clarity.
Cpu utilization rises slightly from 0.6% (idle LibreOffice) to 25% on one core at (or just above) minimum clock frequency (0.48 GHz) (LibreOffice while one button and its tooltip are flashing).

The tooltips shown on hovering the ruler at the top of the page flashes the same way.

Things in the status bar are not affected (they don't flash). (the bar at the bottom of the window, where zoom, page layout, cursor line&column are). Also, the '»' buttons that show the remainder of a toolbar which is too long to fit the screen, is not affected.