[201_104] Fix : Shift+Tab results in empty popup window on >#2726

[Dylan-end]

Fixes #2726
Shift+Tab results in empty popup window on >

Summary

When typing > in math mode and pressing Shift+Tab to cycle backwards through variants, the Tab cycling popup window appears empty instead of showing the expected symbol variants.
Developer Document: devel/201_104.md

The backwards cycling (Shift+Tab) for the > key fails to display any variant in the popup.

Root Cause

The keyboard event handling for Shift+Tab in math mode does not correctly trigger the variant cycling mechanism for the > symbol. Specifically:

1.Shift+Tab is treated as an independent key combination and is not correctly mapped to the "previous variant" operation in the math editing context.
2.When '>' is the first symbol in the variant menu and the user presses Shift+Tab, the system's handling method is to convert it into multiple 'Tab' presses and iterate through < + n 'Tab's to check for existence, until it reaches the last symbol in the variant menu.
3.In the initial definition of < + 6 Tab presses, the last entry consists of two characters and requires special handling. However, such handling is currently only implemented for math-bracket-open. Therefore, < + 6 Tab presses should be changed to math-bracket-open.

As a result, the popup window is empty, and the user cannot see any selectable options.

Changes

In ~/mogan/TeXmacs/progs/math/math-kbd.scm, simply change the definition of < + 6 Tab presses to use math-bracket-open.

How to test

1. Open Mogan Editor.
2. Enter math mode by pressing $.
3. Type > (greater-than sign).
4. Expected behavior before fix: Pressing Shift+Tab shows an empty popup window (or no popup at all).
5. Press Shift+Tab repeatedly to cycle backwards through variants.
6. Verify: The popup window displays the correct variants in order.

Before (Problem)

• When pressing Shift+Tab on >, the popup window is completely empty.

After (Fix)

• Shift+Tab now shows the variant list correctly.

Additional Notes

• "Only one file actually needs to be changed; the other changes were added while I was debugging."

[Dylan-end]

Update: The uploaded files have already been formatted.