When running under Java 1.6, it seems to be impossible to select existing text or paste text into the Header Manager header/value fields. Similarly for Cookie Manager etc. This works fine on Java 1.4 (and sort of works on Java 1.5 - need to double- click in the extreme left-hand of the field) The Name: and Comments: fields work OK in Java 1.6.
This is due to the code in the HeaderPanel.focusLost() method - when the code is disabled, the fields behave OK for editting (and cut and paste). However, the code seems to be necessary to ensure that any text so far entered is saved when the user moves off the field. Further investigation needed...
Found that CookieManager was working OK in 1.6; comparison of this with the HeaderManager showed that the fix was simple - just add a call to stopCellEditing() in modifyTestElement(). The updated code will be in the nightlies after r521876.
See also bug 42948
This issue has been migrated to GitHub: https://github.com/apache/jmeter/issues/1893