[201_90] fix inner product symbol display in tab cycle popup

[1sh-repalto]

Fixes #2926
Inner product symbol does not show on Tab cycling popup window

Summary

When cycling through math variants for the < key (which includes the inner product ⟨⟩ as the 7th variant), the popup window showed a broken/red symbol instead of the angle brackets.

Developer Document: devel/201_90.md

Issue Found

The math-bracket-open case in lambda-to-symbol was concatenating the left and right bracket symbols (e.g., "<langle><rangle>"). This produced a compound string that the widget-box rendering engine (using the roman font) could not render correctly, showing as a red broken symbol or --.

Changes

In TeXmacs/progs/math/math-edit.scm:

Modified lambda-to-symbol to return only the left bracket symbol for math-bracket-open:

 ((math-bracket-open)
  (and (>= (length (cdr body)) 2)
       (string? (cadr body))
       (string? (caddr body))
       (let ((lb (cadr body))
             (rb (caddr body)))
         `(symbol-completion
-          ,(string-append lb rb)))))
+          ,lb))))

How to test

1. Open Mogan Editor
2. Enter math mode by pressing $
3. Type <
4. Press Tab repeatedly (7 times) to reach the inner product ⟨⟩ variant
5. Verify that the popup window shows the left angle bracket ⟨ clearly (instead of a broken red symbol or --)
6. Press Tab once more to return to < and verify the cycle continues correctly

[JackYansongLi]

Could you make it show $\langle \rangle$ instead of a single $\langle$

[wumoin]

The Tab cycling for the `[` symbol seems to have some issues.

[1sh-repalto]

Hello @JackYansongLi and @wumoin ,
Thank you for your reviews. I noticed that these issues had already been fixed when I worked on #2910. I created the PR #2916 which fixed these tab cycling behaviors and it was merged. However, a later PR #2918 modified font_fn in src/Texmacs/Window/tm_button.cpp:

- font fn= smart_font (family, fn_class, series, shape, sz, dpi);
+ font fn= find_font (family, fn_class, series, shape, sz, dpi);

From my investigation, this change led to the unusual behaviour of the fixes I had done in my PR.
From my perspective, the best and minimal change would be to revert find_font back to smart_font. I also attempted to resolve the issue while keeping find_font, but I was unable to find a clean solution. Is there some good reason to let it stay as find_font and also possible to fix the issues at the same time ?

Let me know what you guys think on this.

I have tested both behaviours and am attaching them for reference:

1. with smart_font

with_smart_font.mp4

2. with find_font

with_find_font.mp4