commit f5bbadbad92d55d480ea8b20c1e29f9ffe4ca224
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Thu Jul 14 01:02:28 2022 +0200

    Implement undo coalescing
    
    if the undo element we want to add only changes stuff that was already
    modified by the previous one on undo stack (in the same group), then
    skip it. There is nothing to gain in adding it to the stack.
    
    The typical use case is when doing a search and replace in a large
    document that does many replacements in each paragraph. In this case,
    the same paragraph would be stored repeatedly.
    
    Fixes bug #12564.
---
 src/Undo.cpp | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/src/Undo.cpp b/src/Undo.cpp
index 4e970df5a0..ddc7025e1c 100644
--- a/src/Undo.cpp
+++ b/src/Undo.cpp
@@ -312,12 +312,29 @@ void Undo::Private::doRecordUndo(UndoKind kind,
 
        if (first_pit > last_pit)
                swap(first_pit, last_pit);
+       pit_type const from = first_pit;
+       pit_type const end = cell.lastpit() - last_pit;
+
+       /* Undo coalescing: if the undo element we want to add only
+        * changes stuff that was already modified by the previous one on
+        * undo stack (in the same group), then skip it. There is nothing
+        * to gain in adding it to the stack. The code below works for
+        * both texted and mathed.
+       */
+       if (!stack.empty()
+           && stack.top().group_id == group_id_
+           && !stack.top().bparams
+           && samePar(stack.top().cell, cell)
+           //&& stack.top().kind == kind // needed?
+           && stack.top().from <= from
+           && stack.top().end >= end) {
+               LYXERR(Debug::UNDO, "Undo coalescing: skip entry");
+               return;
+       }
 
        // Undo::ATOMIC are always recorded (no overlapping there).
        // As nobody wants all removed character appear one by one when undoing,
        // we want combine 'similar' non-ATOMIC undo recordings to one.
-       pit_type from = first_pit;
-       pit_type end = cell.lastpit() - last_pit;
        if (!undo_finished_
            && kind != ATOMIC_UNDO
            && !stack.empty()
-- 
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