Try to preserve window position relative to their outputs during hotplug

Currently, if an output is hotplugged, all windows will be scrambled,
which is highly annoying.

With this change, windows will stick to their outputs if an output has
been connected or disconnected.

BUG: 296673
BUG: 378896
BUG: 412703
BUG: 443698

src/abstract_client.cpp

@@ -3346,8 +3346,10 @@ void AbstractClient::checkWorkspacePosition(QRect oldGeometry, QRect oldClientGe
     }
     enum { Left = 0, Top, Right, Bottom };
     const int border[4] = { borderLeft(), borderTop(), borderRight(), borderBottom() };
+
+    QRect newGeom = moveResizeGeometry();
     if( !oldGeometry.isValid())
-        oldGeometry = moveResizeGeometry();
+        oldGeometry = newGeom;
     if (!oldClientGeometry.isValid())
         oldClientGeometry = oldGeometry.adjusted(border[Left], border[Top], -border[Right], -border[Bottom]);
     if (isFullScreen()) {
@@ -3387,20 +3389,18 @@ void AbstractClient::checkWorkspacePosition(QRect oldGeometry, QRect oldClientGe
     // Old and new maximums have different starting values so windows on the screen
     // edge will move when a new strut is placed on the edge.
     QRect oldScreenArea;
+    QRect screenArea;
     if( workspace()->inUpdateClientArea()) {
         // we need to find the screen area as it was before the change
-        oldScreenArea = QRect( 0, 0, workspace()->oldDisplayWidth(), workspace()->oldDisplayHeight());
-        int distance = INT_MAX;
-        const auto previousSizes = workspace()->previousScreenSizes();
-        for (const QRect &r : previousSizes) {
-            int d = r.contains( oldGeometry.center()) ? 0 : ( r.center() - oldGeometry.center()).manhattanLength();
-            if( d < distance ) {
-                distance = d;
-                oldScreenArea = r;
-            }
+        oldScreenArea = workspace()->previousScreenSizes().value(output());
+        if (oldScreenArea.isNull()) {
+            oldScreenArea = output()->geometry();
         }
+        screenArea = output()->geometry();
+        newGeom.translate(screenArea.topLeft() - oldScreenArea.topLeft());
     } else {
         oldScreenArea = workspace()->clientArea(ScreenArea, kwinApp()->platform()->outputAt(oldGeometry.center()), oldDesktop);
+        screenArea = workspace()->clientArea(ScreenArea, this, newGeom.center());
     }
     const QRect oldGeomTall = QRect(oldGeometry.x(), oldScreenArea.y(), oldGeometry.width(), oldScreenArea.height());   // Full screen height
     const QRect oldGeomWide = QRect(oldScreenArea.x(), oldGeometry.y(), oldScreenArea.width(), oldGeometry.height());   // Full screen width
@@ -3408,12 +3408,10 @@ void AbstractClient::checkWorkspacePosition(QRect oldGeometry, QRect oldClientGe
     int oldRightMax = oldScreenArea.x() + oldScreenArea.width();
     int oldBottomMax = oldScreenArea.y() + oldScreenArea.height();
     int oldLeftMax = oldScreenArea.x();
-    const QRect screenArea = workspace()->clientArea(ScreenArea, this, frameGeometry().center());
     int topMax = screenArea.y();
     int rightMax = screenArea.x() + screenArea.width();
     int bottomMax = screenArea.y() + screenArea.height();
     int leftMax = screenArea.x();
-    QRect newGeom = frameGeometry();
     QRect newClientGeom = newGeom.adjusted(border[Left], border[Top], -border[Right], -border[Bottom]);
     const QRect newGeomTall = QRect(newGeom.x(), screenArea.y(), newGeom.width(), screenArea.height());   // Full screen height
     const QRect newGeomWide = QRect(screenArea.x(), newGeom.y(), screenArea.width(), newGeom.height());   // Full screen width