RFR: 8252236: TabPane: must keep header of selected tab visible

[Jeanette Winzenburg]

On Mon, 14 Sep 2020 09:12:12 GMT, Jeanette Winzenburg <fastegal at openjdk.org> wrote:

>> In a bid to find more scenarios like you did mention description, I observed two more scenarios when selected tab does
>> not stay in view. 1. Inserting tabs before or removing tabs after selected tab or moving the selected tab, causes the
>> selected tab to go out of view. 2. Rotating TabPane can also take the selected tab out of view. I am little doubtful if
>> it a legitimate issue. The rotations I tried clipped TabPane and TabPane header.  I think rotation issue can be kept
>> out of scope of this issue, can be filed as a follow on. I leave to it to you to take a call.
>> About 1, I did quickly verify that it gets fixed by adding `invalidateScrollOffset();` call in
>> `TabHeaderArea.removeTab()`, `TabHeaderArea.addTab()` and  `TabHeaderArea.moveTab()`. Also If we fix it here, then bug
>> summary might need a change. It currently reflects only initial tab.
>
> good catch and many thanks for the evaluation - now all I'll have to do is to write tests to catch them :) Will also
> update the bug summary as you suggest.
> Agree, that the second can be regarded as off scope, as it seems to effect both header and content - will not do
> anything in this fix.

Widened scope of the issue to include problems after modifying the list of tabs: added fix and tests.

Separated dragging issues into a new bug (there's an unrelated isssue when dragging over the leading edge, fixing that
might have effects on the trailing edge, so didn't do much to fix the issue/s on the latter except ensuring the
visibility after drop).

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PR: https://git.openjdk.java.net/jfx/pull/300