[LyX/2.3.x] Skip paint event when in the middle of a buffer operation

[Jean-Marc Lasgouttes]

commit 567e94347a4d3c0f191ab731aceba2dda47f28ee
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Thu May 31 23:15:40 2018 +0200

    Skip paint event when in the middle of a buffer operation
    
    This is detected when an undo group is open and contains at east one
    element. This means indeed that changes are in progress. Note that the
    group is in general opened in GuiApplication::dispatch. The code there
    is changed to ensure that the group is closed before updating the
    screen.
    
    This patch is experimental. It is expected to be replaced in master by
    a more complete solution. It could in the meantime be backported to 2.3.x.
    
    Fixes bug #11159.
    
    (cherry picked from commit c7496a11b2f0bd714b6c2ee0f7189ff420e014ce)
    (cherry picked from commit 4d0c43f9aa944649d4b5788c5de98d37c280a036)
---
 src/Undo.cpp                         |    8 ++++++++
 src/Undo.h                           |    2 ++
 src/frontends/qt4/GuiApplication.cpp |   12 +++++++-----
 src/frontends/qt4/GuiWorkArea.cpp    |   10 ++++++----
 4 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/src/Undo.cpp b/src/Undo.cpp
index 5f112b8..33ab923 100644
--- a/src/Undo.cpp
+++ b/src/Undo.cpp
@@ -604,6 +604,14 @@ void Undo::endUndoGroup(CursorData const & cur_after)
 }
 
 
+bool Undo::activeUndoGroup() const
+{
+       return d->group_level_ > 0
+               && !d->undostack_.empty()
+               && d->undostack_.top().group_id == d->group_id_;
+}
+
+
 void Undo::recordUndo(CursorData const & cur, UndoKind kind)
 {
        d->recordUndo(kind, cur, cur.pit(), cur.pit(), cur);
diff --git a/src/Undo.h b/src/Undo.h
index b1908a2..6f6cca6 100644
--- a/src/Undo.h
+++ b/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);
+       /// return true if an undo group is open and contains at least one 
element
+       bool activeUndoGroup() const;
 
        /// The general case: record undo information for an arbitrary range.
        /**
diff --git a/src/frontends/qt4/GuiApplication.cpp 
b/src/frontends/qt4/GuiApplication.cpp
index c65d39e..93eb2e8 100644
--- a/src/frontends/qt4/GuiApplication.cpp
+++ b/src/frontends/qt4/GuiApplication.cpp
@@ -1396,16 +1396,18 @@ DispatchResult const & 
GuiApplication::dispatch(FuncRequest const & cmd)
                
current_view_->currentBufferView()->cursor().saveBeforeDispatchPosXY();
                buffer = &current_view_->currentBufferView()->buffer();
        }
-       // This handles undo groups automagically
-       UndoGroupHelper ugh(buffer);
 
        DispatchResult dr;
+       dr.screenUpdate(Update::FitCursor);
+       {
+               // This handles undo groups automagically
+               UndoGroupHelper ugh(buffer);
+               dispatch(cmd, dr);
+       }
+
        // redraw the screen at the end (first of the two drawing steps).
        // This is done unless explicitly requested otherwise
-       dr.screenUpdate(Update::FitCursor);
-       dispatch(cmd, dr);
        updateCurrentView(cmd, dr);
-
        d->dispatch_result_ = dr;
        return d->dispatch_result_;
 }
diff --git a/src/frontends/qt4/GuiWorkArea.cpp 
b/src/frontends/qt4/GuiWorkArea.cpp
index d68185c..5835cbd 100644
--- a/src/frontends/qt4/GuiWorkArea.cpp
+++ b/src/frontends/qt4/GuiWorkArea.cpp
@@ -1245,10 +1245,12 @@ void GuiWorkArea::Private::paintPreeditText(GuiPainter 
& pain)
 void GuiWorkArea::paintEvent(QPaintEvent * ev)
 {
        // Do not trigger the painting machinery if we are not ready (see
-       // bug #10989). However, since macOS has turned the screen black at
-       // this point, our backing store has to be copied to screen.
-       if (view().busy()) {
-               // this is a no-op except on macOS.
+       // bug #10989). The second test triggers when in the middle of a
+       // dispatch operation.
+       if (view().busy() || 
d->buffer_view_->buffer().undo().activeUndoGroup()) {
+               // Since macOS has turned the screen black at this point, our
+               // backing store has to be copied to screen (this is a no-op
+               // except on macOS).
                d->updateScreen(ev->rect());
                ev->accept();
                return;