ScrollView: Defer calls to doLayout()

doLayout() updates the dimensions of the scroll view, and therefore can
trigger the scroll bars to be shown or hidden. That, in turn, can
trigger another call to doLayout(), resulting in binding loops. Avoid
those by deferring the call.

The scrollview tests are broken without this change because the binding
loop prevents the scrollbars from being shown or hidden in some cases.
The deferring necessitates a tryVerify(), but fixes the actual tests.

Change-Id: I0dac0ba380240e025237f69c4d357d764a535c3d
Fixes: QTBUG-50605
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

2 files changed, 25 insertions, 19 deletions

diff --git a/src/controls/Private/ScrollViewHelper.qml b/src/controls/Private/ScrollViewHelper.qml
index 810de91d..53050108 100644
--- a/src/controls/Private/ScrollViewHelper.qml
+++ b/src/controls/Private/ScrollViewHelper.qml
@@ -68,31 +68,29 @@ Item {
 
     anchors.fill: parent
 
-    property bool recursionGuard: false
-
-    function doLayout() {
-        if (!recursionGuard) {
-            recursionGuard = true
+    Timer {
+        id: layoutTimer
+        interval: 0;
+        onTriggered: {
             blockUpdates = true;
             scrollHelper.contentWidth = flickableItem !== null ? flickableItem.contentWidth : 0
             scrollHelper.contentHeight = flickableItem !== null ? flickableItem.contentHeight : 0
             scrollHelper.availableWidth = viewport.width
             scrollHelper.availableHeight = viewport.height
             blockUpdates = false;
-            recursionGuard = false
         }
     }
 
     Connections {
         target: viewport
-        onWidthChanged: doLayout()
-        onHeightChanged: doLayout()
+        onWidthChanged: layoutTimer.running = true
+        onHeightChanged: layoutTimer.running = true
     }
 
     Connections {
         target: flickableItem
-        onContentWidthChanged: doLayout()
-        onContentHeightChanged: doLayout()
+        onContentWidthChanged: layoutTimer.running = true
+        onContentHeightChanged: layoutTimer.running = true
         onContentXChanged: {
             hscrollbar.flash()
             vscrollbar.flash()
diff --git a/tests/auto/controls/data/tst_scrollview.qml b/tests/auto/controls/data/tst_scrollview.qml
index 42398115..d3bfac4b 100644
--- a/tests/auto/controls/data/tst_scrollview.qml
+++ b/tests/auto/controls/data/tst_scrollview.qml
@@ -212,21 +212,27 @@ TestCase {
         bigItem.height = 100
         bigItem.width = 100
 
-        verify(!scrollView.__horizontalScrollBar.visible, "Scrollbar showing when contents already fit")
-        verify(!scrollView.__verticalScrollBar.visible, "Scrollbar showing when contents already fit")
+        tryVerify(function() { return !scrollView.__horizontalScrollBar.visible }, 50,
+                  "Scrollbar showing when contents already fit")
+        tryVerify(function() { return !scrollView.__verticalScrollBar.visible }, 50,
+                  "Scrollbar showing when contents already fit")
 
         bigItem.height = 1000
         bigItem.width = 1000
 
-        verify(scrollView.__horizontalScrollBar.visible, "Scrollbar not showing when contents are too big")
-        verify(scrollView.__verticalScrollBar.visible, "Scrollbar not showing when contents are too big")
+        tryVerify(function() { return scrollView.__horizontalScrollBar.visible }, 50,
+                  "Scrollbar not showing when contents are too big")
+        tryVerify(function() { return scrollView.__verticalScrollBar.visible }, 50,
+                  "Scrollbar not showing when contents are too big")
 
         //always off
         bigItem.height = 1000
         scrollView.verticalScrollBarPolicy = Qt.ScrollBarAlwaysOff
-        verify(!scrollView.__verticalScrollBar.visible, "Scrollbar showing when disabled")
+        tryVerify(function() { return !scrollView.__verticalScrollBar.visible }, 50,
+                  "Scrollbar showing when disabled")
         bigItem.height = 100
-        verify(!scrollView.__verticalScrollBar.visible, "Scrollbar showing when disabled")
+        tryVerify(function() { return !scrollView.__verticalScrollBar.visible }, 50,
+                  "Scrollbar showing when disabled")
 
         //always on
         scrollView.verticalScrollBarPolicy = Qt.ScrollBarAlwaysOn
@@ -258,12 +264,14 @@ TestCase {
         verify(scrollView !== null, "view created is null")
         verify(scrollView.flickableItem.contentY === 0)
 
+        tryVerify(function() { return scrollView.__verticalScrollBar.visible });
+
         mouseClick(scrollView, scrollView.width -2, scrollView.height/2, Qt.LeftButton)
-        verify(Math.round(scrollView.flickableItem.contentY) === 100)
+        tryVerify(function() { return Math.round(scrollView.flickableItem.contentY) === 100 });
 
-        verify(scrollView.flickableItem.contentX === 0)
+        tryVerify(function() { return scrollView.flickableItem.contentX === 0 })
         mouseClick(scrollView, scrollView.width/2, scrollView.height - 2, Qt.LeftButton)
-        verify(Math.round(scrollView.flickableItem.contentX) === 100)
+        tryVerify(function() { return Math.round(scrollView.flickableItem.contentX) === 100 })
     }
 
     function test_viewport() {