-moz-background-clip: padding syntax was only supported until Firefox 3.6 according to https://web.archive.org/web/20130827100732/https://developer.mozilla.org/en-US/docs/Web/CSS/background-clip. Firefox 4.0 released 03/2011 supports standard background-clip: padding-box.

The same docs also imply that -webkit-background-clip: padding-box currently used by Phorge was never supported by browsers. Per https://caniuse.com/mdn-css_properties_background-clip_padding-box, Chrome 4 released 10/2010 supports standard background-clip: padding-box.

Compare current webroot/rsrc/css/phui/phui-image-mask.css which does not have this old vendor-prefixed syntax.

• Open the Conpherence top bar dropdown, select "Persistent Chat", see that its .jx-scrollbar-handle based scrollbar still works as expected.
• Read CSS docs. Probably install ancient browser versions if you don't trust documentation. Try https://jigsaw.w3.org/css-validator/ and don't spot issues.