Allow to undo partly math autocorrect

To this end, introduce Undo::splitUndoGroup, which ends currently
group and creates a new one with same nesting level.

src/Cursor.cpp

@@ -607,6 +607,12 @@ void CursorData::endUndoGroup() const
 }
 
 
+void CursorData::splitUndoGroup() const
+{
+	buffer()->undo().splitUndoGroup(*this);
+}
+
+
 void CursorData::recordUndo(pit_type from, pit_type to) const
 {
 	buffer()->undo().recordUndo(*this, from, to);

src/Cursor.h

@@ -183,6 +183,8 @@ public:
 	void beginUndoGroup() const;
 	/// end the current undo group
 	void endUndoGroup() const;
+	/// end abruptly the current group and create a new one wih the same nesting level
+	void splitUndoGroup() const;
 
 	/// The general case: prepare undo for an arbitrary range.
 	void recordUndo(pit_type from, pit_type to) const;

src/Undo.cpp

@@ -610,6 +610,16 @@ void Undo::endUndoGroup(CursorData const & cur_after)
 }
 
 
+void Undo::splitUndoGroup(CursorData const & cur)
+{
+	size_t const level = d->group_level_;
+	d->group_level_ = 1;
+	endUndoGroup(cur);
+	beginUndoGroup(cur);
+	d->group_level_ = level;
+}
+
+
 bool Undo::activeUndoGroup() const
 {
 	return d->group_level_ > 0

src/Undo.h

@@ -96,6 +96,8 @@ public:
 	void endUndoGroup();
 	/// end the current undo group and set UndoElement::cur_after if necessary.
 	void endUndoGroup(CursorData const & cur_after);
+	/// end abruptly the current group and create a new one wih the same nesting level
+	void splitUndoGroup(CursorData const & cur);
 	/// return true if an undo group is open and contains at least one element
 	bool activeUndoGroup() const;
 

src/mathed/MathAutoCorrect.cpp

@@ -90,8 +90,17 @@ bool Correction::correct(Cursor & cur, char_type c) const
 	LYXERR(Debug::MATHED, "match found! subst in " << cur.cell()
 		<< " from: '" << from1_ << "' to '" << to_ << '\'');
 
-	cur.cell().erase(cur.pos() - n, cur.pos());
-	cur.pos() -= n;
+	/* To allow undoing the completion, we proceed in 4 steps
+	 * - inset the raw character
+	 * - split undo group so that we have two separate undo actions
+	 * - record undo, delete the character we just entered and the from1_ part
+	 * - finally, do the insertion of the correction.
+	 */
+	cur.insert(c);
+	cur.splitUndoGroup();
+	cur.recordUndoSelection();
+	cur.cell().erase(cur.pos() - n - 1, cur.pos());
+	cur.pos() -= n + 1;
 
 	cur.insert(to_);
 	return true;