Following your suggestion every thing worked fine.

gemglfw3window is created and works.

03.texture.sharing example open 2 context but texture is rendered only in the first one. Hovewer there is a comment in the patch saying "on Linux, this is only supported by [gemglxwindow] and [gemglutwindow]."

So, I guess still is not working with gemglfw3window.

BTW gemglutwindow  doesn't work properly with sharing context. This time the b context is rendered as a diagonal glitched line while "a" context is rendered as aspected.

thanks for your time and develop, is very apreciated

husk