[vim/vim] helptoc navigation jumps incorrectly and flickers when hlsearch is active and using J/K (Issue #21051)

VimWei created an issue (vim/vim#21051)

**Issue Description:**
When active search highlighting (`hlsearch`) is present in the buffer, navigating the `helptoc` popup menu via `J` / `K` causes screen flickering and incorrect cursor movement when changing heading depth levels.

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**Steps to Reproduce:**

1. Create a Markdown file (e.g., `test.md`) with hierarchical headers:
```markdown
# level 1 a
## level 2 b
### level 3 c
### level 3 d
### level 3 e
### level 3 f

```


2. Open the file in GVIM and trigger search highlighting (e.g., `/level` and press `Enter`). Ensure `hlsearch` is active.
3. Open the table of contents popup via `:HelpToc`.
4. Press uppercase `J` or `K` to navigate through the popup entries.

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**Observed Behavior:**

1. **Window Flickering:** Every keypress of `J` or `K` causes the `helptoc` popup window to flicker/redraw.
2. **First Jump Lag on Depth Change:** When moving between different heading levels (e.g., from `## level 2 b` down to `### level 3 c`):
* **1st press of `J`:** The cursor remains stuck at `## level 2 b`.
* **2nd press of `J`:** The cursor finally jumps to `### level 3 c`.


3. **Unexpected Cursor Retreat:** Continuing to press `J` to move from `### level 3 c` down to `### level 3 d` causes the window to flicker and the cursor to incorrectly jump back up to `## level 2 b`.

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**Expected Behavior:**
`helptoc` navigation using `J` / `K` should smoothly move between heading items regardless of active search highlighting (`hlsearch`), without screen flickering or incorrect level jumps.

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**Environment:**

* **OS:** Windows 10
* **Vim Version:** gVim 9.2.0907 x64 (signed)
* **Terminal/GUI:** gVim (GUI)

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