RFR: 8263095: Provide a way for a custom control to indicate that its userAgentStyleSheet has changed

[Kevin Rushforth]

On Mon, 31 May 2021 13:57:22 GMT, Alessadro Parisi <github.com+16880178+palexdev at openjdk.org> wrote:

> This change allows custom control to change their style dynamically
> When the user-agent stylesheet changes the property automatically calls `NodeHelper.reapplyCSS(Region.this);` to recompute the CSS for the node and its children.
> To make this work the StyleManager class must be fixed too.
> The line `regionUserAgentStylesheet = weakRegionUserAgentStylesheetMap.computeIfAbsent((Region)region, Region::getUserAgentStylesheet);
> ` is problematic because if the region is already present in the map but the user-agent stylesheet changed, the value of `regionUserAgentStylesheet` is not updated and this leads to controls having the wrong style.
> To fix this issue I added this check:
> 
> if (((Region) region).getUserAgentStylesheet() != null && !((Region) region).getUserAgentStylesheet().equals(regionUserAgentStylesheet)) {
>     weakRegionUserAgentStylesheetMap.put((Region) region, ((Region) region).getUserAgentStylesheet());
>     regionUserAgentStylesheet = ((Region) region).getUserAgentStylesheet();
> }

Moving this to `Draft` to make it clear that it isn't yet ready for review.

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PR: https://git.openjdk.java.net/jfx/pull/525