Keep degenerate UIA text ranges degenerate after movement (#7530)

Conhost expands UIA text ranges when moved. This means that degenerate
ranges become non-degenerate after movement, leading to odd behaviour
from UIA clients. This PR doesn't expand degenerate ranges, but rather
keeps them degenerate by moving `_end` to the newly-changed `_start`.

Tested in the NVDA Python console (cases with `setEndPoint` and
`compareEndPoints` described in #7342). Also ran the logic by
@michaelDCurran.

Closes #7342

Almost definitely addresses nvaccess/nvda#11288 (although I'll need to
test with my Braille display). Also fixes an issue privately reported to
me by @Simon818 with copy/paste from review cursor which originally lead
me to believe the issue was with `moveEndPointByRange`.

src/interactivity/win32/ut_interactivity_win32/UiaTextRangeTests.cpp

@@ -583,7 +583,7 @@ class UiaTextRangeTests
                 {
                     5,
                     {4 , 0 + 1},
-                    {5 , 0 + 1}
+                    {4 , 0 + 1}
                 }
             },
 
@@ -694,6 +694,30 @@ class UiaTextRangeTests
                     {0, bottomRow},
                     {0, bottomRow}
                 }
+            },
+
+            MoveTest{
+                L"can move to a new row when necessary when moving forward",
+                { lastColumnIndex, 0 },
+                { lastColumnIndex, 0 },
+                5,
+                {
+                    5,
+                    {0, 0 + 5},
+                    {0, 0 + 5}
+                }
+            },
+
+            MoveTest{
+                L"can move to a new row when necessary when moving backward",
+                { 0, 7 },
+                { 0, 7 },
+                -5,
+                {
+                    -5,
+                    {0, 7 - 5},
+                    {0, 7 - 5}
+                }
             }
         };
         // clang-format on

src/types/UiaTextRangeBase.cpp

@@ -579,6 +579,7 @@ IFACEMETHODIMP UiaTextRangeBase::Move(_In_ TextUnit unit,
     // We can abstract this movement by moving _start, but disallowing moving to the end of the buffer
     constexpr auto endpoint = TextPatternRangeEndpoint::TextPatternRangeEndpoint_Start;
     constexpr auto preventBufferEnd = true;
+    const auto wasDegenerate = IsDegenerate();
     try
     {
         if (unit == TextUnit::TextUnit_Character)
@@ -603,8 +604,17 @@ IFACEMETHODIMP UiaTextRangeBase::Move(_In_ TextUnit unit,
     // If we actually moved...
     if (*pRetVal != 0)
     {
-        // then just expand to get our _end
-        ExpandToEnclosingUnit(unit);
+        if (wasDegenerate)
+        {
+            // GH#7342: The range was degenerate before the move.
+            // To keep it that way, move _end to the new _start.
+            _end = _start;
+        }
+        else
+        {
+            // then just expand to get our _end
+            ExpandToEnclosingUnit(unit);
+        }
     }
 
     UiaTracing::TextRange::Move(unit, count, *pRetVal, *this);