[LyX/master] Do not allow nesting macro names in macro names

[Enrico Forestieri]

commit 1ac2dc208238a28c95275117e2bc90ea651bdf36
Author: Enrico Forestieri <forenr at lyx.org>
Date:   Mon Nov 21 18:24:29 2022 +0100

    Do not allow nesting macro names in macro names
    
    LyX removes a single backslash when it is inserted in a
    macro name by the keyboard. However if it is followed by
    another character before moving the cursor, it is retained
    causing an assertion (bug #12601).
---
 src/mathed/InsetMathNest.cpp |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/src/mathed/InsetMathNest.cpp b/src/mathed/InsetMathNest.cpp
index 70ef185..673a62d 100644
--- a/src/mathed/InsetMathNest.cpp
+++ b/src/mathed/InsetMathNest.cpp
@@ -1825,9 +1825,15 @@ bool InsetMathNest::interpretChar(Cursor & cur, char_type const c)
 		//lyxerr << "starting with macro" << endl;
 		bool reduced = cap::reduceSelectionToOneCell(cur);
 		if (reduced || !cur.selection()) {
+			InsetMath const * im = cur.inset().asInsetMath();
+			InsetMathMacro const * macro = im ? im->asMacro()
+			                                  : nullptr;
+			bool in_macro_name = macro
+				&& macro->displayMode() ==
+					InsetMathMacro::DISPLAY_UNFOLDED;
 			cur.recordUndoInset();
 			docstring const safe = cap::grabAndEraseSelection(cur);
-			if (!cur.inRegexped())
+			if (!cur.inRegexped() && !in_macro_name)
 				cur.insert(MathAtom(new InsetMathUnknown(from_ascii("\\"), safe, false)));
 			else
 				cur.niceInsert(createInsetMath("backslash", buf));