I just installed this build on Win 10 64 bit https://binary-factory.kde.org/view/Windows%2064-bit/job/Dolphin_Release_win64/lastSuccessfulBuild/artifact/dolphin-19.08.0-518-windows-msvc2017_64-cl.exe Close Window button is greyed out. Right click - Close (Alt+F4) is greyed out too. The only way I can close the window is right click on Dolphin in the task bar - Close Window.
*** This bug has been marked as a duplicate of bug 413025 ***