Sorry for the late reply, but I've been quite busy lately. So, the code change that I suggested here #525 <https://github.com/openjdk/jfx/pull/525> is not only an improvement but also a bug fix.
The improvement is that we can see this change as a little theming system because if a control can provide an userAgentStylesheet depending on certain conditions we can call this a theme system. This would be a good improvement for devs like me who develop custom controls because as of now the only way to provide such mechanism is to change the stylesheet list. Let's say we have an enumerator for the style of a control and each enum has the string url to the CSS file, the only way is to load the CSS and do 'control.getStylesheets().setAll(file)' This, however, can cause nasty bugs and unexpected behaviors. That's why I have overridden the 'invalidated()' method of the property, so that when it changes, the control's style is re-computed correctly. Regarding the 'StyleManager' the changes ensure that what's stored in the 'weakRegionUserAgentStylesheetMap' is the correct CSS file. In fact, using just the 'computeIfAbsent()' method here Line 1667 <https://github.com/openjdk/jfx/pull/525/commits/3cffaccd7eaf11238e786e31f8c1b6fb16121471#diff-ca9344a33e4b346c9ac138a5e5d867acc3b0e53c761e8d207fd92026320792edR1667> is not enough if the user agent changes. The good thing about the user agent API change is that it is backward compatible, libraries based on old JavaFX versions would still work because as of now they override the 'getUserAgentStylesheet()' getter, and that getter is still present but by default now returns the userAgenStylesheet property value. Let me know what do you think, I would really like to see this change happen, implementing custom themes would be very easy
