Color of vertical shadow in frametitle, beamer, warsaw

I know that's its a common topic, but still I don't get it to work. I want a different background color (due to corporate design) and shadows which fade to that new color.

I have checked several threads, where the shadow is removed (that's not what i want) or where the shadow color is changed manually.

Latex beamer block shadow fades to white and not to the background color

-> The solution has no effect on the title-shadow

How to add a transparent shadow to a block in beamer?

-> I changed the beamerbaseboxes.sty, but it had no effect (I refreshed the FNDB and updated the formats after replacing the file)

My minimal example:

\documentclass{beamer}
\usetheme{Warsaw}
\setbeamercolor{background canvas}{bg=yellow}

\begin{document}
\begin{frame}{Nice Title}
Stupid Content
\end{frame}
\end{document}


The result is (I added arrows to emphasize the shadow):

I expect the vertical shadow to start from black and end at yellow, like the background is.

Best regards, karl.ranseier

This bug was fixed in https://github.com/josephwright/beamer/commit/c7051f7003834fb06e40e97288de03f1364163f1 which will probably be included in beamer v3.51

\documentclass{beamer}
\usetheme{Warsaw}
\setbeamercolor{background canvas}{bg=yellow}

\AtBeginDocument{