Looks like Opera revamped the sidebar.
Something changed in Opera, I'll have to look into it and see if I can accommodate it.
I have no idea whats the problem but I'll try to look into it.

The new sidebar is too restrictive and breaking some sidebar extensions.
"window.close()" doesn't work with it.
It doesn't seem to respect tab.update's tab focusing.

For now disable this flag if your sidebar extension is not behaving right:
opera://flags/#sidebar-extensions-in-browser-sidebar

@cratte Click the isometric box icon on the toolbar and pin SimpleUndoClose back onto the toolbar.

@cratte Click the isometric box icon on the toolbar and pin SimpleUndoClose back onto the toolbar.

@themoisterymachine Can you give me a link to the dark theme plugin?

The new sidebar is too restrictive and breaking some sidebar extensions.
"window.close()" doesn't work with it.
It doesn't seem to respect tab.update's tab focusing.

For now disable this flag if your sidebar extension is not behaving right:
opera://flags/#sidebar-extensions-in-browser-sidebar

Great that theres a flag to revert back to the old sidebar.

Took a look at the new sidebar and it seems restrictive.
The "window.close()" method I use to close the sidebar doesn't work with the new implementation.
Setting the tab to focus on bookmark left-click (and therefore closing sidebar) doesn't work too.
Though it seems like that behavior is hard-coded by Opera on middle-click.
Whether the new sidebar predicatively close or stay is up to the pin up top right.
Its all restricted and I can't seem to get my way with how the new sidebar behave.

Looks like Opera revamped the sidebar.
Something changed in Opera, I'll have to look into it and see if I can accommodate it.
I have no idea whats the problem but I'll try to look into it.

@vsto22010: I have no such problem on my side.
The bookmarks bar stay hidden for me.

@vsto22010: You can drag any link to any folder name in Bookmarks by the Side, the title text will become bold. Release the drag and the "New bookmark" dialog will slide in to confirm the save.