Created attachment 2359 [details] screenshot showing the problem I configured Claws to have the folder tree on the left, the message list on the top right and the message view on the bottom right. When using fancy HTML view, it adds buttons below the message view, taking up a whole line of vertical screen space. In my desktop resolution (1920x1080) it would be better if the icons were on the right side - that would not take so much vertical space, and leave more for the message view itself.
As an alternative, you could simply make the message view taller.
Moving such tool's menu to the side isn't something easily achievable for the plugin, as far as I know. That's outside of the plugin's viewer scope. Too much effort for little gain.