[vim/vim] Latest expanded status line breaks plugins like lightline and airline (Issue #20948)

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dougaak created an issue (vim/vim#20948)

### Steps to reproduce

1. Start a latest version of Vim and install status line plugins, I've tested lightline and airline in their default setup
2. `:vsplit | wincmd w`
3. Toggle some mode changes to make the status line changes, e.g. press keys: i -> Esc -> v -> Esc. You shall see that the cell under the v-split border is having incorrect color, the color of the last status line update.

<img width="754" height="186" alt="Image" src="https://github.com/user-attachments/assets/37ef865a-5c16-4057-a5a6-cd7d588e3681" />
<img width="754" height="186" alt="Image" src="https://github.com/user-attachments/assets/2139dbca-2187-42fc-b5b6-880d70995914" />
<img width="754" height="186" alt="Image" src="https://github.com/user-attachments/assets/831cca97-281b-4a7c-9b20-f4a1f8ecdb3b" />
<img width="754" height="186" alt="Image" src="https://github.com/user-attachments/assets/060080b2-3089-42ff-a9fb-3babdcfd51ed" />
<img width="754" height="186" alt="Image" src="https://github.com/user-attachments/assets/6e9e3c9a-d38b-4f67-826b-1139ee7348ce" />

I'll add that the incorrect cell color will fix itself once you typed another key

### Expected behaviour

The expanded status line cell under the v-split border should updates its value timely to the update of the status line. I.e., back to the previous mode sequence, the status line under the v-split border should always looks like these in any situation:
<img width="944" height="186" alt="Image" src="https://github.com/user-attachments/assets/f35b792f-cc34-48a8-9dda-cc9a8e6f8db1" />
<img width="944" height="186" alt="Image" src="https://github.com/user-attachments/assets/7fefc383-f212-43e5-8d4e-78f3b0730519" />
<img width="944" height="186" alt="Image" src="https://github.com/user-attachments/assets/1c8d80be-72da-4ded-b969-19d3b0c1c1ee" />

Or at least there is a configuration to disable this status line expansion feature.

### Version of Vim

9.2.0907

### Environment

OS: Windows 11
Terminal: Windows Terminal, also able to reproduce in gVim

### Logs and stack traces

```shell

```

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<div style=3D"display: flex; flex-wrap: wrap; white-space: pre-wrap; align-=
items: center; "><img height=3D"20" width=3D"20" style=3D"border-radius:50%=
; margin-right: 4px;" decoding=3D"async" src=3D"https://avatars.githubuserc=
ontent.com/u/34620532" /><strong>dougaak</strong> created an issue <a href=
=3D"https://github.com/vim/vim/issues/20948">(vim/vim#20948)</a></div>
<h3 dir=3D"auto">Steps to reproduce</h3>
<ol dir=3D"auto">
<li>Start a latest version of Vim and install status line plugins, I've tes=
ted lightline and airline in their default setup</li>
<li><code class=3D"notranslate">:vsplit | wincmd w</code></li>
<li>Toggle some mode changes to make the status line changes, e.g. press ke=
ys: i -&gt; Esc -&gt; v -&gt; Esc. You shall see that the cell under the v-=
split border is having incorrect color, the color of the last status line u=
pdate.</li>
</ol>
<a href=3D"https://github.com/user-attachments/assets/37ef865a-5c16-4057-a5=
a6-cd7d588e3681">Image: Image (view on web)</a>
<a href=3D"https://github.com/user-attachments/assets/2139dbca-2187-42fc-b5=
b6-880d70995914">Image: Image (view on web)</a>
<a href=3D"https://github.com/user-attachments/assets/831cca97-281b-4a7c-9b=
20-f4a1f8ecdb3b">Image: Image (view on web)</a>
<a href=3D"https://github.com/user-attachments/assets/060080b2-3089-42ff-a9=
fb-3babdcfd51ed">Image: Image (view on web)</a>
<a href=3D"https://github.com/user-attachments/assets/6e9e3c9a-d38b-4f67-82=
6b-1139ee7348ce">Image: Image (view on web)</a>
<p dir=3D"auto">I'll add that the incorrect cell color will fix itself once=
 you typed another key</p>
<h3 dir=3D"auto">Expected behaviour</h3>
<p dir=3D"auto">The expanded status line cell under the v-split border shou=
ld updates its value timely to the update of the status line. I.e., back to=
 the previous mode sequence, the status line under the v-split border shoul=
d always looks like these in any situation:<br>
<a href=3D"https://github.com/user-attachments/assets/f35b792f-cc34-48a8-9d=
da-cc9a8e6f8db1">Image: Image (view on web)</a><br>
<a href=3D"https://github.com/user-attachments/assets/7fefc383-f212-43e5-8d=
4e-78f3b0730519">Image: Image (view on web)</a><br>
<a href=3D"https://github.com/user-attachments/assets/1c8d80be-72da-4ded-b9=
69-19d3b0c1c1ee">Image: Image (view on web)</a></p>
<p dir=3D"auto">Or at least there is a configuration to disable this status=
 line expansion feature.</p>
<h3 dir=3D"auto">Version of Vim</h3>
<p dir=3D"auto">9.2.0907</p>
<h3 dir=3D"auto">Environment</h3>
<p dir=3D"auto">OS: Windows 11<br>
Terminal: Windows Terminal, also able to reproduce in gVim</p>
<h3 dir=3D"auto">Logs and stack traces</h3>
<div class=3D"highlight highlight-source-shell" dir=3D"auto"><pre class=3D"=
notranslate"></pre></div>

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