When rendering a macro, the conversionContext that is passed in to the macro's execute method can be used to determine in what context the macro is being rendered and, thus, what kind of output the macro should produce.

The full set of possible output types is enumerated in the ConversionContextOutputType enum in Confluence's com.atlassian.confluence.content.render.xhtml package.

Unfortunately, the PREVIEW value is not set when previewing a page. The output type is still set to DISPLAY in this case. The output type is set to PREVIEW when rendering in the macro browser.

A macro may wish to know when it is being rendered in preview mode in order to dumb-down any interactive functionality that is only intended to work when actually viewing the page, rather than previewing it.

I haven't checked to see how far back this bug exists, but it is likely that it was either introduced in Confluence 4.0, or has existed since around the Confluence 2.x timeframe.