QSplitterHandle: Ensure a minimum grab area

This used to work only for 0- and 1-pixel wide handles. However,
and paradoxically, 2- and 3-pixel wide handles would end up with
narrower grab areas. We now ensure a 4 or 5 minimum grab area,
depending on the handle width parity.

The patch also clears the margins and mask if the handle size is
increased at some point.

Change-Id: I8a16e39fb34b5452d9021dbde8c22bec79df0243
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

src/widgets/widgets/qsplitter.cpp

@@ -241,21 +241,24 @@ void QSplitterHandle::resizeEvent(QResizeEvent *event)
 {
     Q_D(const QSplitterHandle);
 
-    // When splitters are only 1 or 0 pixel large we increase the
-    // actual grab area to five pixels
+    // Ensure the actual grab area is at least 4 or 5 pixels
+    const int handleMargin = (5 - d->s->handleWidth()) / 2;
 
     // Note that QSplitter uses contentsRect for layouting
     // and ensures that handles are drawn on top of widgets
     // We simply use the contents margins for draggin and only
     // paint the mask area
-    bool useTinyMode = (d->s->handleWidth() <= 1);
+    const bool useTinyMode = handleMargin > 0;
     setAttribute(Qt::WA_MouseNoMask, useTinyMode);
     if (useTinyMode) {
         if (orientation() == Qt::Horizontal)
-            setContentsMargins(2, 0, 2, 0);
+            setContentsMargins(handleMargin, 0, handleMargin, 0);
         else
-            setContentsMargins(0, 2, 0, 2);
+            setContentsMargins(0, handleMargin, 0, handleMargin);
         setMask(QRegion(contentsRect()));
+    } else {
+        setContentsMargins(0, 0, 0, 0);
+        clearMask();
     }
 
     QWidget::resizeEvent(event);

tests/auto/widgets/widgets/qsplitter/tst_qsplitter.cpp

@@ -81,6 +81,7 @@ private slots:
     void replaceWidget();
     void replaceWidgetWithSplitterChild_data();
     void replaceWidgetWithSplitterChild();
+    void handleMinimumWidth();
 
     // task-specific tests below me:
     void task187373_addAbstractScrollAreas();
@@ -820,6 +821,29 @@ void tst_QSplitter::replaceWidgetWithSplitterChild()
     }
 }
 
+void tst_QSplitter::handleMinimumWidth()
+{
+    MyFriendlySplitter split;
+    split.addWidget(new QLabel("Number Wan"));
+    split.addWidget(new QLabel("Number Too"));
+
+    split.show();
+    QTest::qWaitForWindowExposed(&split);
+    for (int i = 0; i < 10; i++) {
+        split.setHandleWidth(i);
+        QTest::qWait(100); // resizing
+        QCOMPARE(split.handle(1)->width(), qMax(4 + (i & 1), i));
+    }
+
+    split.setOrientation(Qt::Vertical);
+    QTest::qWait(100);
+    for (int i = 0; i < 10; i++) {
+        split.setHandleWidth(i);
+        QTest::qWait(100); // resizing
+        QCOMPARE(split.handle(1)->height(), qMax(4 + (i & 1), i));
+    }
+}
+
 void tst_QSplitter::rubberBandNotInSplitter()
 {
     MyFriendlySplitter split;