[LyX/2.3.x] Remember correctly pixel ratio used for painting

[Jean-Marc Lasgouttes]

commit c43b6a9ecf7d78a2187384e776d439698633f01f
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Mon Sep 18 10:58:07 2017 +0200

    Remember correctly pixel ratio used for painting
    
    This avoids endless resize issues on HiDPI systems (e.g. Retina Mac).
    
    Rename pixel_ratio_ to last_pixel_ratio_ to emphasize that this is a
    cached value.
    
    Inline needResize method to make the logic clearer in paintEvent.
    
    (cherry picked from commit 6532e5104dfad5416817d89a5f91e53c30cdd523)
---
 src/frontends/qt4/GuiWorkArea.cpp       |    6 ++++--
 src/frontends/qt4/GuiWorkArea_Private.h |    6 +-----
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/src/frontends/qt4/GuiWorkArea.cpp 
b/src/frontends/qt4/GuiWorkArea.cpp
index 6520c3a..f4380da 100644
--- a/src/frontends/qt4/GuiWorkArea.cpp
+++ b/src/frontends/qt4/GuiWorkArea.cpp
@@ -238,7 +238,7 @@ GuiWorkArea::Private::Private(GuiWorkArea * parent)
 : p(parent), buffer_view_(0), lyx_view_(0),
   caret_(0), caret_visible_(false),
   need_resize_(false), preedit_lines_(1),
-  pixel_ratio_(1.0),
+  last_pixel_ratio_(1.0),
   completer_(new GuiCompleter(p, p)), dialog_mode_(false), 
shell_escape_(false),
   read_only_(false), clean_(true), externally_modified_(false)
 {
@@ -1243,7 +1243,7 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
        // LYXERR(Debug::PAINTING, "paintEvent begin: x: " << rc.x()
        //      << " y: " << rc.y() << " w: " << rc.width() << " h: " << 
rc.height());
 
-       if (d->needResize()) {
+       if (d->need_resize_ || pixelRatio() != d->last_pixel_ratio_) {
                d->resizeBufferView();
                if (d->caret_visible_) {
                        d->hideCaret();
@@ -1251,6 +1251,8 @@ void GuiWorkArea::paintEvent(QPaintEvent * ev)
                }
        }
 
+       d->last_pixel_ratio_ = pixelRatio();
+
        GuiPainter pain(viewport(), pixelRatio());
        d->buffer_view_->draw(pain, d->caret_visible_);
 
diff --git a/src/frontends/qt4/GuiWorkArea_Private.h 
b/src/frontends/qt4/GuiWorkArea_Private.h
index 8be2869..ef2bb7e 100644
--- a/src/frontends/qt4/GuiWorkArea_Private.h
+++ b/src/frontends/qt4/GuiWorkArea_Private.h
@@ -97,10 +97,6 @@ struct GuiWorkArea::Private
 
        void paintPreeditText(GuiPainter & pain);
 
-       bool needResize() const {
-               return need_resize_ || p->pixelRatio() != pixel_ratio_;
-       }
-
        ///
        GuiWorkArea * p;
        ///
@@ -133,7 +129,7 @@ struct GuiWorkArea::Private
        /// Ratio between physical pixels and device-independent pixels
        /// We save the last used value to detect changes of the
        /// current pixel_ratio of the viewport.
-       double pixel_ratio_;
+       double last_pixel_ratio_;
        ///
        GuiCompleter * completer_;