Fix colors getting lost on reflow (#17568)

The "copy the remaining attributes" loop assumes that it has full
ownership over the rows that it copies. For that to be true,
we have to of course make sure that the current write-cursor
is at a fresh, new row in the first place.

## Validation Steps Performed
* In a new pwsh tab with 120 colums:
  ``Write-Host -NoNewline "`e[36m$('a'*120)`e[m"; sleep 10``
* Resize the window wider
* Color doesn't get lost

src/buffer/out/textBuffer.cpp

@@ -2855,6 +2855,15 @@ void TextBuffer::Reflow(TextBuffer& oldBuffer, TextBuffer& newBuffer, const View
         }
     }
 
+    // The for loop right after this if condition will copy entire rows of attributes at a time.
+    // This assumes of course that the "write cursor" (newX, newY) is at the start of a row.
+    // If we didn't check for this, we may otherwise copy attributes from a later row into a previous one.
+    if (newX != 0)
+    {
+        newX = 0;
+        newY++;
+    }
+
     // Finish copying buffer attributes to remaining rows below the last
     // printable character. This is to fix the `color 2f` scenario, where you
     // change the buffer colors then resize and everything below the last

src/buffer/out/ut_textbuffer/ReflowTests.cpp

@@ -589,13 +589,13 @@ namespace
                 TestBuffer{
                     { 2, 5 }, // reduce width aggressively
                     {
-                        { L"  ", false },
                         { L"  ", true },
                         { L"  ", true },
                         { L"  ", true },
                         { L"  ", true },
+                        { L"  ", false },
                     },
-                    { 1, 0 },
+                    { 1, 4 },
                 },
             },
         },