sw: fix bug with redline following fieldmark or hidden break

It is now possible for HideIterator to move to another node without a
redline being involved, but if this happens and there was no redline on
the previous node, GetRedlinePos() had already returned
SwRedlineTable::npos and now no redlines can be found in the new node.

(regression from commit 657de5fba1)

Change-Id: I536b2660c97501be896203403cdd5e2f2015e816
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/181946
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>

sw/qa/extras/layout/layout2.cxx

@@ -959,6 +959,19 @@ CPPUNIT_TEST_FIXTURE(SwLayoutWriter2, testHiddenParaProps)
     CPPUNIT_ASSERT_EQUAL(
         SvxAdjust::Left,
         pTextFrame->GetTextNodeForParaProps()->GetSwAttrSet().Get(RES_PARATR_ADJUST).GetAdjust());
+
+    dispatchCommand(mxComponent, u".uno:ShowTrackedChanges"_ustr, {});
+
+    pTextFrame = dynamic_cast<SwTextFrame*>(pBody->GetLower());
+    for (int i = 0; i < 18; ++i)
+    {
+        pTextFrame = dynamic_cast<SwTextFrame*>(pTextFrame->GetNext());
+    }
+    // the problem was that this redline (following hidden) wasn't merged
+    CPPUNIT_ASSERT_EQUAL(u"10 visible, hidden-merge, visible, delete-merge, visible"_ustr,
+                         pTextFrame->GetText());
+    pTextFrame = dynamic_cast<SwTextFrame*>(pTextFrame->GetNext());
+    CPPUNIT_ASSERT_EQUAL(u"abcdefghi"_ustr, pTextFrame->GetText());
 }
 
 CPPUNIT_TEST_FIXTURE(SwLayoutWriter2, testTdf151954)

sw/source/core/text/redlnitr.cxx

@@ -78,6 +78,7 @@ private:
     /// current start/end pair
     SwPosition const* m_pStartPos;
     SwPosition const* m_pEndPos;
+    SwNode const* m_pCurrentRedlineNode;
 
 public:
     SwPosition const* GetStartPos() const { return m_pStartPos; }
@@ -95,6 +96,7 @@ public:
         , m_RedlineIndex(isHideRedlines ? m_rIDRA.GetRedlinePos(rTextNode, RedlineType::Any) : SwRedlineTable::npos)
         , m_pStartPos(nullptr)
         , m_pEndPos(&m_Start)
+        , m_pCurrentRedlineNode(&rTextNode)
     {
     }
 
@@ -108,6 +110,14 @@ public:
         assert(m_pEndPos);
         if (m_isHideRedlines)
         {
+            // GetRedlinePos() returns npos if there is no redline on the
+            // node but something else could have merged nodes so search again!
+            if (m_RedlineIndex == SwRedlineTable::npos
+                && &m_pEndPos->GetNode() != m_pCurrentRedlineNode)
+            {
+                m_RedlineIndex = m_rIDRA.GetRedlinePos(m_pEndPos->GetNode(), RedlineType::Any);
+                m_pCurrentRedlineNode = &m_pEndPos->GetNode();
+            }
             // position on current or next redline
             for (; m_RedlineIndex < m_rIDRA.GetRedlineTable().size(); ++m_RedlineIndex)
             {