Support for right-to-left languages

[Caleb Rouleau]

Hi,

I really appreciate QScintilla. Thanks for writing it and maintaining it!

I was wondering what the state of support for right-to-left languages like
Arabic and Hebrew should be in QScintilla. (I don't see any mention of
support in the documentation but I may be missing it!) I'm also wondering
whether you plan to work to improve this support in the future. I see on
the Scintilla website (1) that Scintilla has "limited experimental" support
for these languages on Windows. I read this thread on GitHub (2) that shows
that this support might be desirable for other projects than just mine.

Here's an issue that I'm having with right-to-left languages:
1. On a Windows 10 machine, install Hebrew or Arabic language support.
2. Build the example application that comes with QScintilla against Qt 5.15
and QScintilla 2.14.0 and run it.
3. Use Windows UI to switch your keyboard to Hebrew or Arabic.
4. Type "asdf" on your keyboard into the example application. Since your
keyboard is in Arabic or Hebrew mode, this should type "شسيب" or "שדגכ".
However, no text is visible.
5. Switch your keyboard back to English.
6. Type "asdf". Suddenly the Arabic or Hebrew text appears and you should
see something like "שדגכasdf"

Is there something that I can do differently to make the right-to-left text
appear immediately instead of only showing when there is also left-to-right
text in the same box?

It seems likely that this issue happens on other versions of Qt and
versions of Windows as well, but I haven't tested it yet.

Thank you!

Caleb

(1) https://www.scintilla.org/ScintillaDoc.html
(2) https://github.com/mu-editor/mu/issues/752
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