<Swing Dev> [10] RFR JDK-7190581: Nimbus: PgDn at the bottom causes scrolling

[Prasanta Sadhukhan]

Hi All,

Please review a fix for an issue where it is seen that pressing "Page 
down" after "ctrl-END" in TextArea causes extra scroll down for Nimbus L&F.
ctrl-END is supposed to go the very end of the document and show the 
caret at the end, so any "page down" should not cause any further scroll 
down.

This was happening because JTextArea uses 
DefaultEditorKit.VerticalPageAction() to execute pagedown/up and
due to container layout a bit different in different l&fs, in 
ViewportLayout.layoutContainer causes "viewSize" to be different as set 
in vp.setViewSize(viewSize);
So, the "initialY" view point as obtained from 
VerticalPageAction#target.getVisibleRect();   is different in Nimbus l&f 
as compared to say, Metal l&f.
When constrainY() is called to ascertain if "initialY" is valid, it 
calculates new Y again [using Math.max(0, target.getHeight() - vis);]
which causes "newVis.y" to be different so 
target.scrollRectToVisible(newVis); at the end with new visual y , 
causes scrolling.

In other l&f like metal, initialY and newVis.y are same so no additional 
scrolling is seen.

Proposed fix is to ensure if caret position [obtained from 
target.getCaretPosition()] is same as visible offset,
which means caret is at the end of the document when "page down" is 
pressed after ctrl-END
or
at the start when "page up" is pressed after ctrl-HOME
then correct the newVis.y to prevent additional scrolling.

Bug: https://bugs.openjdk.java.net/browse/JDK-7190581
webrev: http://cr.openjdk.java.net/~psadhukhan/7190581/webrev.00/

I have tested with Notepad demo which uses DefaultEditorKit with 
pageup/down actions and also other regression tests which uses 
DefaultEditorKit.

Regards
Prasanta