I ended up restoring an earlier version of the opera app from Time Machine and disabling automatic updates. To disable, open a terminal window and enter this command:
$ defaults write com.operasoftware.Opera OPDisableAutoUpdate true
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If Opera is running with no open windows, clicking the dock icon won't open a new browser window. I feel this changed recently since I don't recall ever needing to use the new window command (command-N) to open a window. In comparison, Firefox and Safari will open a new window automatically under the same conditions. Running High Sierra 10.13.6 & Opera 58.0.3135.132. Is there a setting to change this behavior?