Adjuncts do not include CSS correctly for removable dom fragments

[stephenc]

• I think it only works by accident with script tags
• When the adjunct is initially included the script and css elements are injected at the point where the st:adjunct
  tag is first referenced.
• Thus these tags form part of the DOM that can be removed (e.g. when part of a repeatable / hetero list)
• Because the script is executed, removing the DOM node cannot un-execute the script, so script tags are safe
• Removing the css's link tag, however, removes the styling... and the adjunct manager doesn't know.
• What this hack does is inject the CSS at the end of the DOM's head tag (yes triggering a reflow) but only
  if the CSS link is not present already. The JavaScript tag (because we are bypassing st:adjunct) is conditionally
  appended to the DOM's body so that it can be fetched while other scripts continue executing.

To see the bug this fixes:

• Create a job with the GIT SCM and a couple of SCM locations and some credential parameters
• Save the job
• Load the job
• Remove the first credential parameter, now all the credential tags loose the styling (width:auto on the drop-down) and the Add button overflows to the next line

[stephenc]

@kohsuke here's a strange one for you!

[stephenc]

@reviewbybees

[jglick]

Ideally this could be proven with a functional test, but I am not sure if HtmlUnit is powerful enough to deal with the dynamic DOM changes.

This should be fixed upstream in Stapler.

[kohsuke]

Created JENKINS-26578 to track the fix in stapler. A comment should be added that this fix is a temporary work around until that gets resolved.

I'm OK with pushing this fix in, until core picks up JENKINS-26578 whenever that happens.

[rsandell]

+1