Do not try to rearrange if the width/height is < 0

This also fixes a crash in this peculiar condition:

Item {
    width: 0
    height: 0
    RowLayout {
        anchors.fill: parent
        anchors.leftMargin: 1
        anchors.leftMargin: 1
    }
}

Reason for crash:
As a consequence the layout was rearranged with size == (-1, -1), (which
happens to be the value of uninitialized QSizeF).

This invalid size was passed down to QGridLayoutEngine::setGeometries()
(in the contentsGeometry QRect), and again to ensureGeometries().
ensureGeometries() would return early because of the condition

    if (q_cachedSize == size)

(q_cachedSize is an uninitialized QSizeF by default, and size == (-1, -1)

This caused the q_xx, q_yy, q_widths, and q_height to not be properly
initialized, and it would crash inside
QGridLayoutEngine::setGeometries() when accessing the list:

    qreal x = q_xx.at(item->firstColumn());

Pick-to: 6.2 6.5 6.7
Fixes: QTBUG-124456
Change-Id: I2e3586389aa1728c3622e92bd589af87d11955ae
Reviewed-by: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>

1 files changed, 43 insertions, 0 deletions

diff --git a/tests/auto/quick/qquicklayouts/data/tst_rowlayout.qml b/tests/auto/quick/qquicklayouts/data/tst_rowlayout.qml
index 03c186fdb4..d31a0b0fb8 100644
--- a/tests/auto/quick/qquicklayouts/data/tst_rowlayout.qml
+++ b/tests/auto/quick/qquicklayouts/data/tst_rowlayout.qml
@@ -1579,6 +1579,49 @@ Item {
             compare(rootRect.item1.width, 100)
         }
 
+        //---------------------------
+        // Layout with negative size
+        Component {
+            id: negativeSize_Component
+            Item {
+                id: rootItem
+                width: 0
+                height: 0
+                // default width x height: (0 x 0)
+                RowLayout {
+                    spacing: 0
+                    anchors.fill: parent
+                    anchors.leftMargin: 1   // since parent size == (0 x 0), it causes layout size
+                    anchors.bottomMargin: 1 // to become (-1, -1)
+                    Item {
+                        Layout.fillWidth: true
+                        Layout.fillHeight: true
+                    }
+                }
+            }
+        }
+
+        function test_negativeSize() {
+            let rootItem = createTemporaryObject(negativeSize_Component, container)
+            let rowLayout = rootItem.children[0]
+            let item = rowLayout.children[0]
+
+            const arr = [7, 1, 7, 0]
+            arr.forEach((n) => {
+                                rootItem.width = n
+                                rootItem.height = n
+
+                                // n === 0 is special: It will cause the layout to have a
+                                // negative size. In this case it will simply not rearrange its
+                                // child (and leave it at its previous size, 6)
+                                const expectedItemExtent = n === 0 ? 6 : n - 1
+
+                                compare(item.width, expectedItemExtent)
+                                compare(item.height, expectedItemExtent)
+                               });
+        }
+
+
 //---------------------------
         Component {
             id: rowlayoutWithTextItems_Component