(This is related to the sidebar added in Opera Developer 29.0.1794.0)
Different sidebar extensions require different width of the sidebar. Every time I switch extension in the sidebar I have to resize it so it fits with the extension, or so it's not unnecessary wide.
What I would like you to do, is to make it save the width of it for each extension, so when you switch extension, the width changes automatically.
Indeed, a state with saved width for each extension would be nice. Otherwise an option to resize from within API, each developer would then save width and adjust it, but first option would be more convenient.
Sorry for up but I agree with you, the sidebar api necessary need something to resize the sidebar from content script.