[LyX/2.3.x] Improve the logic of caret repainting

[Jean-Marc Lasgouttes]

commit 6bd158dcba851beb4c22f8aa79a572b21f1a16d0
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Fri Sep 29 10:29:39 2017 +0200

    Improve the logic of caret repainting
    
    For some reason the existing code only considered the bottom row that
    contained the cursor. There is no need for that, and actually it
    caused painting problems in nested insets.
    
    Tweak the logic of repaint_caret_row_ a bit: there is no need for
    repainting when there is currently no caret on screen.
    
    (cherry picked from commit 764a153c69bb9b46a6e6872ce48e06f5f867cc53)
---
 src/BufferView.cpp |   18 +++++++++---------
 1 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/BufferView.cpp b/src/BufferView.cpp
index 58e4f86..53d374f 100644
--- a/src/BufferView.cpp
+++ b/src/BufferView.cpp
@@ -3057,15 +3057,15 @@ void BufferView::draw(frontend::Painter & pain, bool 
paint_caret)
        int const y = tm.first().second->position();
        PainterInfo pi(this, pain);
 
-       CursorSlice const & bottomSlice = d->cursor_.bottom();
-       /**  A repaint of the previous cursor row is needed if
-        * 1/ the caret will be painted and is is not the same than the
-        *    already painted one;
-        * 2/ the caret will not be painted, but there is already one on
-        * screen.
+       /**  A repaint of the previous caret row is needed if there is
+        *  caret painted on screen and either
+        *   1/ a new caret has to be painted at a place different from
+        *      the existing one;
+        *   2/ there is no need for a caret anymore.
         */
-       d->repaint_caret_row_ = (paint_caret && bottomSlice != d->caret_slice_)
-               || (! paint_caret && !d->caret_slice_.empty());
+       d->repaint_caret_row_ = !d->caret_slice_.empty() &&
+               ((paint_caret && d->cursor_.top() != d->caret_slice_)
+                || ! paint_caret);
 
        // Check whether the row where the cursor lives needs to be scrolled.
        // Update the drawing strategy if needed.
@@ -3151,7 +3151,7 @@ void BufferView::draw(frontend::Painter & pain, bool 
paint_caret)
 
        // Remember what has just been done for the next draw() step
        if (paint_caret)
-               d->caret_slice_ = bottomSlice;
+               d->caret_slice_ = d->cursor_.top();
        else
                d->caret_slice_ = CursorSlice();
 }