fix(space): check edges for nearly complete screen overlap

When two outputs are positioned on top of each other in a way that one covers
most of the height or width of the onter one with exception of a small gap this
gap might be too small to later on substract the orner offset and stay
positive. In this case do not create the edge at all.

The screen edges test passes again.

screenedge.cpp

@@ -1068,7 +1068,10 @@ void ScreenEdges::createVerticalEdge(ElectricBorder border, const QRect &screen,
         const ElectricBorder edge = (border == ElectricLeft) ? ElectricBottomLeft : ElectricBottomRight;
         m_edges << createEdge(edge, x, screen.y() + screen.height() -1, 1, 1);
     }
-    // create border
+    if (height <= m_cornerOffset) {
+        // An overlap with another output is near complete. We ignore this border.
+        return;
+    }
     m_edges << createEdge(border, x, y, 1, height);
 }
 
@@ -1088,6 +1091,10 @@ void ScreenEdges::createHorizontalEdge(ElectricBorder border, const QRect &scree
         // also right most edge
         width -= m_cornerOffset;
     }
+    if (width <= m_cornerOffset) {
+        // An overlap with another output is near complete. We ignore this border.
+        return;
+    }
     const int y = (border == ElectricTop) ? screen.y() : screen.y() + screen.height() - 1;
     m_edges << createEdge(border, x, y, width, 1);
 }
@@ -1099,6 +1106,10 @@ Edge *ScreenEdges::createEdge(ElectricBorder border, int x, int y, int width, in
 #else
     Edge *edge = kwinApp()->platform()->createScreenEdge(this);
 #endif
+    // Edges can not have negative size.
+    Q_ASSERT(width >= 0);
+    Q_ASSERT(height >= 0);
+
     edge->setBorder(border);
     edge->setGeometry(QRect(x, y, width, height));
     if (createAction) {