RFR: JDK-8314215 Trailing Spaces before Line Breaks Affect the Center Alignment of Text [v3]

[Andy Goryachev]

On Fri, 26 Jan 2024 01:27:50 GMT, John Hendrikx <jhendrikx at openjdk.org> wrote:

>> There are a number of tickets open related to text rendering:
>> 
>> https://bugs.openjdk.org/browse/JDK-8314215
>> 
>> https://bugs.openjdk.org/browse/JDK-8145496
>> 
>> https://bugs.openjdk.org/browse/JDK-8129014
>> 
>> They have in common that wrapped text is taking the trailing spaces on each wrapped line into account when calculating where to wrap.  This looks okay for text that is left aligned (as the spaces will be trailing the lines and generally aren't a problem, but looks weird with CENTER and RIGHT alignments.  Even with LEFT alignment there are artifacts of this behavior, where a line like `AAA  BBB  CCC` (note the **double** spaces) gets split up into `AAA  `, `BBB  ` and `CCC`, but if space reduces further, it will wrap **too** early because the space is taken into account (ie. `AAA` may still have fit just fine, but `AAA  ` doesn't, so the engine wraps it to `AA` + `A  ` or something).
>> 
>> The fix for this is two fold; first the individual lines of text should not include any trailing spaces into their widths; second, the code that is taking the trailing space into account when wrapping should ignore all trailing spaces (currently it is ignoring all but one trailing space).  With these two fixes, the layout in LEFT/CENTER/RIGHT alignments all look great, and there is no more early wrapping due to a space being taking into account while the actual text still would have fit (this is annoying in tight layouts, where a line can be wrapped early even though it looks like it would have fit).
>> 
>> If it were that simple, we'd be done, but there may be another issue here that needs solving: wrapped aligned TextArea's.
>> 
>> TextArea don't directly support text alignment (via a setTextAlignment method like Label) but you can change it via CSS.
>> 
>> For Left alignment + wrapping, TextArea will ignore any spaces typed before a line that was wrapped.  In other words, you can type spaces as much as you want, and they won't show up and the cursor won't move.  The spaces are all getting appended to the previous line.  When you cursor through these spaces, the cursor can be rendered out of the control's bounds.  To illustrate, if you have the text `AAA                 BBB CCC`, and the text gets wrapped to `AAA`, `BBB`, `CCC`, typing spaces before `BBB` will not show up.  If you cursor back, the cursor may be outside the control bounds because so many spaces are trailing `AAA`.
>> 
>> The above behavior has NOT changed, is pretty standard for wrapped text controls,...
>
> John Hendrikx has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Use Character::isWhitespace to check for breakable spaces
>  - Fix bug with shared positions array (it is not 0 indexed per TextRun)

char     isWhiteSpace (breaking space)?
\u180e   false <- unexpected
\u200b   false <- unexpected
\u200c   false <- unexpected
\u200d   false <- unexpected

I suspect this result is correct:
\u180e   mongolian vowel separator
\u200b   false <- zero-width space
\u200c   false <- zero-width non-joiner (ZWNJ)
\u200d   false <- The right-to-left mark (RLM)

For instance,
https://unicode-explorer.com/c/180E
It is no longer classified as space character (i.e. in Zs category) in Unicode 6.3.0, even though it was in previous versions of the standard.

Here is the (non-authoritative) list:
https://www.fileformat.info/info/unicode/category/Zs/list.htm

Anyway, we probably can rely on `Character.isWhitespace()` being correct.

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PR Comment: https://git.openjdk.org/jfx/pull/1236#issuecomment-1912275984