Ok i will post it later but i went in fullscreen, with my Isil.8no button UI.
Toolbar was set at 0.77 switched to 1.00 scale without relogging.
I could use mounted combat without any problem
and i can use it too at any scale.
Now if i log off and relog with my scale set already to 1 for the toolbar then i can not either click on it.
All is working fine if this is either over 1 or below.
So i have the same problem but never encountered as i dont use the scale 1.
Sorry for the bad leading
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So i can confirme that, loading with an UI at scale above or below 1, IF you change your scale to 1, without going off, then the skills are available. But if you decide de log off, then you need to play again with the scale.
Considering it is more a bug than anything on our side, i will not for now touch any of my settings code for the bars. I will see if players are ok to go either at 1.1 or 0.99 scale for now.