Avoid flicker when invalidate is propagated in a widget/layout hierarchy

* Do not call invalidate from activateRecursive().
  This resulted in that a layout was invalidated as many times as there
  were items in the layout.

* Several improvements. Do not call resize(size()) too often.
  Calling resize() from the widgetEvent() is not very nice though...

* Remove commented out code

* make sure layout is activated even if the widget does not change size

* activate the layout if the resize is same as size()

* In order to not break existing apps, make this an opt-in feature
  with QGraphicsLayout::setInstantInvalidatePropagation(true);

Reviewed-by: Frederik Gladhorn
Reviewed-by: John Tapsell

1 files changed, 4 insertions, 5 deletions

diff --git a/src/gui/graphicsview/qgraphicslinearlayout.cpp b/src/gui/graphicsview/qgraphicslinearlayout.cpp
index 5591638395..1229379ee8 100644
--- a/src/gui/graphicsview/qgraphicslinearlayout.cpp
+++ b/src/gui/graphicsview/qgraphicslinearlayout.cpp
@@ -275,17 +275,16 @@ void QGraphicsLinearLayout::insertItem(int index, QGraphicsLayoutItem *item)
         qWarning("QGraphicsLinearLayout::insertItem: cannot insert itself");
         return;
     }
-    Q_ASSERT(item);
 
-    //the order of the following instructions is very important because
-    //invalidating the layout before adding the child item will make the layout happen
-    //before we try to paint the item
-    invalidate();
     d->addChildLayoutItem(item);
 
     d->fixIndex(&index);
     d->engine.insertRow(index, d->orientation);
     new QGridLayoutItem(&d->engine, item, d->gridRow(index), d->gridColumn(index), 1, 1, 0, index);
+    //the order of the following instructions is very important because
+    //invalidating the layout before adding the child item will make the layout happen
+    //before we try to paint the item
+    invalidate();
 }
 
 /*!