RFR: 8335547: Support multi-line prompt text for TextArea [v4]

[Kevin Rushforth]

On Wed, 26 Mar 2025 14:11:34 GMT, Michael Strauß <mstrauss at openjdk.org> wrote:

>> The tests show that only LF "\n" is rendered as a new line, there is no need to add more restrictions that is not needed
>> and the same was tested by @andy-goryachev-oracle previously in the comments and it confirms the same.
>
> That doesn't sound like a compelling reason to me. In fact, it makes it seems like a bug in JavaFX that a line break is only rendered with `\n`, but not with `\r\n` or `\r`.
> 
> In any case, the goal here is to (semantically) transform a string such that it doesn't contain line breaks, and line breaks come in three different usual forms. Our goal should always be to do the right thing, and not stop half-way and rely on unspecified rendering quirks for the rest.

Option 1 is intentionally the status quo, and matches what Swing's JComponent does, although @mstr2 is right that this isn't documented. An RFE to treat `\r` or `\r\n` as a newline could be considered in the future. We wouldn't do that as part of this PR.

So for _this_ PR, the question is what characters should be elided for the prompt text of a `TextField` so that multiple lines as a single line? Limiting this to stripping `\n` is sufficient given the current implementation, unless and until something else changes. Also, it matches what the existing implementation tries to do when it modifies the actual property value.

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PR Review Comment: https://git.openjdk.org/jfx/pull/1716#discussion_r2019136526