tableViewP->setShowGrid(false) still shows vertical lines

this bug is not fixed in 6.9 beta 2, even though the bug is marked as closed (as of feb 20 2025).

please follow the repro steps below, and you can easily see the issue.

To reproduce:

1. on windows 11, run Qt Creator with Qt 6.9 beta 2
2. open the "Frozen Column Example"
3. in the file `FreezeTableWidget`, go to line 45
4. add the lines `setShowGrid(false);` and `frozenTableView->setShowGrid(false);`
5. run the app (note uses "windows" modern style by default)
6. <-- see screen shot: still shows vertical lines
7. set style to "windowsvista"
8. <-- works correctly (see visa.png)