commit 02e01c1658f27a840a01ce4d6d87b856d5dfcae6
Author: Aleix Pol <aleixpol@kde.org>
Date:   Mon Apr 15 14:31:52 2019 +0100

    Don't rescale every output every time outputs change - DAVID VERSION
    
    Summary:
    Every time a display gets connected/disconnected, every output will be
    reinitialized. We don't really need to reset the scaling to 1 every time
    we don't know, especially since the setting has been set voluntarily.
    
    Test Plan: When I disconnect the external display, my laptop doesn't
    fallback to scale=1.
    
    Reviewers: #plasma
    
    Subscribers: davidedmundson, ngraham, kwin
    
    Tags: #kwin
    
    Differential Revision: https://phabricator.kde.org/D20521

diff --git a/abstract_output.cpp b/abstract_output.cpp
index e2f1598e2..a100bda98 100644
--- a/abstract_output.cpp
+++ b/abstract_output.cpp
@@ -163,6 +163,7 @@ void AbstractOutput::setWaylandMode(const QSize &size, int refreshRate)
         m_xdgOutput->setLogicalSize(pixelSize() / scale());
         m_xdgOutput->done();
     }
+    emit modeChanged();
 }
 
 void AbstractOutput::createXdgOutput()
diff --git a/plugins/platforms/drm/drm_backend.cpp b/plugins/platforms/drm/drm_backend.cpp
index 4f07d8c4c..e43e5ff70 100644
--- a/plugins/platforms/drm/drm_backend.cpp
+++ b/plugins/platforms/drm/drm_backend.cpp
@@ -514,7 +514,8 @@ void DrmBackend::readOutputsConfiguration()
         const auto outputConfig = configGroup.group((*it)->uuid());
         (*it)->setGlobalPos(outputConfig.readEntry<QPoint>("Position", pos));
         // TODO: add mode
-        (*it)->setScale(outputConfig.readEntry("Scale", 1.0));
+        if (outputConfig.hasKey("Scale"))
+            (*it)->setScale(outputConfig.readEntry("Scale", 1.0));
         pos.setX(pos.x() + (*it)->geometry().width());
     }
 }