output changes: handle to-be-enabled outputs first

This prevents situations where we have no enabled outputs

src/backends/drm/drm_backend.cpp

@@ -640,7 +640,8 @@ DrmGpu *DrmBackend::findGpuByFd(int fd) const
 
 bool DrmBackend::applyOutputChanges(const WaylandOutputConfig &config)
 {
-    QVector<DrmOutput*> changed;
+    QVector<DrmOutput*> toBeEnabled;
+    QVector<DrmOutput*> toBeDisabled;
     for (const auto &gpu : qAsConst(m_gpus)) {
         const auto &outputs = gpu->outputs();
         for (const auto &o : outputs) {
@@ -650,10 +651,17 @@ bool DrmBackend::applyOutputChanges(const WaylandOutputConfig &config)
                 continue;
             }
             output->queueChanges(config);
-            changed << output;
+            if (config.constChangeSet(output)->enabled) {
+                toBeEnabled << output;
+            } else {
+                toBeDisabled << output;
+            }
         }
         if (!gpu->testPendingConfiguration()) {
-            for (const auto &output : qAsConst(changed)) {
+            for (const auto &output : qAsConst(toBeEnabled)) {
+                output->revertQueuedChanges();
+            }
+            for (const auto &output : qAsConst(toBeDisabled)) {
                 output->revertQueuedChanges();
             }
             return false;
@@ -661,7 +669,10 @@ bool DrmBackend::applyOutputChanges(const WaylandOutputConfig &config)
     }
     // first, apply changes to drm outputs.
     // This may remove the placeholder output and thus change m_outputs!
-    for (const auto &output : qAsConst(changed)) {
+    for (const auto &output : qAsConst(toBeEnabled)) {
+        output->applyQueuedChanges(config);
+    }
+    for (const auto &output : qAsConst(toBeDisabled)) {
         output->applyQueuedChanges(config);
     }
     // only then apply changes to the virtual outputs
@@ -669,7 +680,7 @@ bool DrmBackend::applyOutputChanges(const WaylandOutputConfig &config)
         if (!qobject_cast<DrmOutput*>(output)) {
             output->applyChanges(config);
         }
-    };
+    }
     if (Compositor::compositing()) {
         Compositor::self()->scene()->addRepaintFull();
     }

src/platform.cpp

@@ -151,8 +151,20 @@ void Platform::requestOutputsChange(KWaylandServer::OutputConfigurationV2Interfa
 
 bool Platform::applyOutputChanges(const WaylandOutputConfig &config)
 {
-    const auto outputs = enabledOutputs();
-    for (const auto &output : outputs) {
+    const auto availableOutputs = outputs();
+    QVector<AbstractOutput*> toBeEnabledOutputs;
+    QVector<AbstractOutput*> toBeDisabledOutputs;
+    for (const auto &output : availableOutputs) {
+        if (config.constChangeSet(qobject_cast<AbstractWaylandOutput*>(output))->enabled) {
+            toBeEnabledOutputs << output;
+        } else {
+            toBeDisabledOutputs << output;
+        }
+    }
+    for (const auto &output : toBeEnabledOutputs) {
+        static_cast<AbstractWaylandOutput*>(output)->applyChanges(config);
+    }
+    for (const auto &output : toBeDisabledOutputs) {
         static_cast<AbstractWaylandOutput*>(output)->applyChanges(config);
     }
     return true;