Handle parent being a child's focus procy in QWidget::setFocusProxy

When a parent became a new child's focus proxy in an existing focus
chain, the child was appended at the end of the chain.
That leads to broken tab order, e.g. with a QComboBox which became
editable after a focus chain has been set.

This patch captures the case and insertes the new child after its
parent in the focus chain.

A corresponding test function is added in tst_QWidget.

Fixes: QTBUG-111978
Pick-to: 6.5
Change-Id: I3a426c0560fa830b7b7ffead54f26dd0adef499f
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 24 insertions, 0 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index cde0542eee..d5dbc3f21a 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -6412,6 +6412,30 @@ void QWidget::setFocusProxy(QWidget * w)
         d->focus_prev = oldPrev;
         oldPrev->d_func()->focus_next = this;
         firstChild->d_func()->focus_prev = this;
+    } else if (w->isAncestorOf(this)) {
+        // If the focus proxy is a parent, 'this' has to be inserted directly after its parent in the focus chain
+        // remove it from the chain and insert this into the focus chain after its parent
+
+        // is this the case already?
+        QWidget *parentsNext = w->d_func()->focus_next;
+        if (parentsNext == this) {
+            // nothing to do.
+            Q_ASSERT(d->focus_prev == w);
+        } else {
+            // Remove 'this' from the focus chain by making prev and next point directly to each other
+            QWidget *myOldNext = d->focus_next;
+            QWidget *myOldPrev = d->focus_prev;
+            if (myOldNext && myOldPrev) {
+                myOldNext->d_func()->focus_prev = myOldPrev;
+                myOldPrev->d_func()->focus_next = myOldNext;
+            }
+
+            // Insert 'this' behind the parent
+            w->d_func()->focus_next = this;
+            d->focus_prev = w;
+            d->focus_next = parentsNext;
+            parentsNext->d_func()->focus_prev = this;
+        }
     }
 
     if (moveFocusToProxy)