RFR: 5080391: ArrayIndexOutOfBounds during "undo" of Right-to-Left text insert [v2]

[Prasanta Sadhukhan]

> javax.swing.text.AbstractDocument$BranchElement.replace(...) method throws an `ArrayIndexOutOfBoundsException: arraycopy: length -1 is negative` when using an UndoManager on the default document of a JTextArea and you try to undo the insertion of a LEFT-TO-RIGHT language (e.g. Arabic) that is immediately followed by setting the component orientation on the JTextArea.
> 
> This is because System.arrayCopy() is called with -ve length because of the calculation done in AbstractDocment.replace where `src` is of 2bytes because of unicode text causing `nmove` to become -ve if `nchildren` is 1 (an unicode character is inserted)
> 
> System.arrayCopy throws `IndexOutOfBoundsException if:
> 
>     The srcPos argument is negative.
>     The destPos argument is negative.
>     The length argument is negative
> 
> 
> so the fix is made to make  nmove, src, dest +ve
> Also, Element.getElement() can return null which is not checked, causing NPE, if deletion of text is done which results in no element, which is also fixed in the PR
> 
> All jtreg testsuite tests are run without any regression.

Prasanta Sadhukhan has updated the pull request incrementally with one additional commit since the last revision:

  Test fix

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10446/files
  - new: https://git.openjdk.org/jdk/pull/10446/files/c1020f15..8e07fdc3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10446&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10446&range=00-01

  Stats: 198 lines in 2 files changed: 114 ins; 84 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10446.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10446/head:pull/10446

PR: https://git.openjdk.org/jdk/pull/10446