- Make a fresh install (clean INI) of XnView from a ZIP package (XnView-win-small.zip or XnView-win.zip or XnView-win-full.zip).
- Start XnView and choose “application folder” as “location for program settings” in the Configuration wizard.
- Go to Tools » Options... » Interface » Toolbar » Buttons » Buttons displayed.
- Click the Default toolbar button.
» 3 buttons are added to the Basic toolbar. - Click the OK button to leave the options dialog.
» A file named default.bar is created in XnView's program directory. - Make a copy of default.bar and rename the copy to custom.bar.
- Go to the toolbar options and click on the ... button in the Buttons displayed field to load the custom.bar presets.
- Make a few random changes to the toolbars and save them by clicking on OK.
- Go back to the toolbar options and click on the Default toolbar button.
» custom.bar is overwritten with the settings of default.bar.
IMO the Default toolbar button should reset the toolbars to the settings specified in default.bar without overwriting any customizations.