Fix positioning and placement of initially invisible items in layouts

Instead of misusing implicitWidth/Height as a storage mechanism in
scenarios where width/height is used for preferred size evaluation,
new members are created to hold this information to prevent incorrect
sizing in mentioned scenario and get rid of blocking signals.

Task-number: QTBUG-57455
Change-Id: I102c45805c4106e5829b17b65c2e247b2381573f
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

2 files changed, 28 insertions, 30 deletions

diff --git a/src/imports/layouts/qquicklayout.cpp b/src/imports/layouts/qquicklayout.cpp
index df64b593d9..eb61eb687f 100644
--- a/src/imports/layouts/qquicklayout.cpp
+++ b/src/imports/layouts/qquicklayout.cpp
@@ -95,6 +95,8 @@ QQuickLayoutAttached::QQuickLayoutAttached(QObject *parent)
       m_maximumWidth(std::numeric_limits<qreal>::infinity()),
       m_maximumHeight(std::numeric_limits<qreal>::infinity()),
       m_defaultMargins(0),
+      m_fallbackWidth(-1),
+      m_fallbackHeight(-1),
       m_row(-1),
       m_column(-1),
       m_rowSpan(1),
@@ -1049,39 +1051,32 @@ void QQuickLayout::effectiveSizeHints_helper(QQuickItem *item, QSizeF *cachedSiz
         prefHeight = qCeil(item->implicitHeight());
 
     // If that fails, make an ultimate fallback to width/height
-
-    if (!info && (prefWidth < 0 || prefHeight < 0))
-        info = attachedLayoutObject(item);
-
-    if (useFallbackToWidthOrHeight && info) {
-        /* This block is a bit hacky, but if we want to support using width/height
-           as preferred size hints in layouts, (which we think most people expect),
-           we only want to use the initial width.
-           This is because the width will change due to layout rearrangement, and the preferred
-           width should return the same value, regardless of the current width.
-           We therefore store the width in the implicitWidth attached property.
-           Since the layout listens to changes of implicitWidth, (it will
-           basically cause an invalidation of the layout), we have to disable that
-           notification while we set the implicit width (and height).
-
-           Only use this fallback the first time the size hint is queried. Otherwise, we might
-           end up picking a width that is different than what was specified in the QML.
+    if (useFallbackToWidthOrHeight && !prefS.isValid()) {
+        /* If we want to support using width/height as preferred size hints in
+           layouts, (which we think most people expect), we only want to use the
+           initial width.
+           This is because the width will change due to layout rearrangement,
+           and the preferred width should return the same value, regardless of
+           the current width.
+           We therefore store this initial width in the attached layout object
+           and reuse it if needed rather than querying the width another time.
+           That means we need to ensure that an Layout attached object is available
+           by creating one if necessary.
         */
-        if (prefWidth < 0 || prefHeight < 0) {
-            item->blockSignals(true);
-            if (prefWidth < 0) {
-                prefWidth = item->width();
-                item->setImplicitWidth(prefWidth);
-            }
-            if (prefHeight < 0) {
-                prefHeight = item->height();
-                item->setImplicitHeight(prefHeight);
-            }
-            item->blockSignals(false);
-        }
-    }
+        if (!info)
+            info = attachedLayoutObject(item);
 
+        auto updatePreferredSizes = [](qreal &cachedSize, qreal &attachedSize, qreal size) {
+            if (cachedSize < 0) {
+                if (attachedSize < 0)
+                    attachedSize = size;
 
+                cachedSize = attachedSize;
+            }
+        };
+        updatePreferredSizes(prefWidth, info->m_fallbackWidth, item->width());
+        updatePreferredSizes(prefHeight, info->m_fallbackHeight, item->height());
+    }
 
     // Normalize again after the implicit hints have been gathered
     expandSize(prefS, minS);
diff --git a/src/imports/layouts/qquicklayout_p.h b/src/imports/layouts/qquicklayout_p.h
index 7984e1bd4d..1e9984e337 100644
--- a/src/imports/layouts/qquicklayout_p.h
+++ b/src/imports/layouts/qquicklayout_p.h
@@ -294,6 +294,9 @@ private:
     qreal m_defaultMargins;
     QMarginsF m_margins;
 
+    qreal m_fallbackWidth;
+    qreal m_fallbackHeight;
+
     // GridLayout specific properties
     int m_row;
     int m_column;