Since I suppose you gave a try to safe mode or with a brand new LO profile, I wonder if it may be related to Skia.
Did you give a try at https://wiki.documentfoundation.org/QA/FirstSteps#Graphics-related_issues_.28Skia.29 ?

(In reply to Julien Nabet from comment #1)
> Since I suppose you gave a try to safe mode or with a brand new LO profile,
> I wonder if it may be related to Skia.
> Did you give a try at
> https://wiki.documentfoundation.org/QA/FirstSteps#Graphics-related_issues_.
> 28Skia.29 ?
If you look at my pasted version information above, you'll see that it was without Skia, shown by "UI render: default;".

But just to be sure, I just checked again with a fresh profile (therefore using Skia), the problem is still there.
Version: 7.1.0.0.beta1 (x64)
Build ID: 828a45a14a0b954e0e539f5a9a10ca31c81d8f53
CPU threads: 2; OS: Windows 10.0 Build 18363; UI render: Skia/Raster; VCL: win
Locale: zh-CN (zh_CN); UI: zh-CN
Calc: threaded

I'd be rather surprised if others on Windows can not reproduce this bug, as I've seen the same problem in other screenshots (for unrelated problems) posted by others here on Bugzilla.

Sorry for the noise, it's indeed in your initial description that you don't use Skia.

confirm with Version: 7.2.0.0.alpha0+ (x64)
Build ID: 1ac449874c0e93ee7e7a9765053d392fde0067f4
CPU threads: 4; OS: Windows 10.0 Build 17763; UI render: Skia/Vulkan; VCL: win
Locale: cs-CZ (cs_CZ); UI: en-US
Calc: CL

bisected with win64-7.1
 c4c985a67e8f30635ae1fa34e028392eaaa31888 is the first bad commit
commit c4c985a67e8f30635ae1fa34e028392eaaa31888
Author: Norbert Thiebaud <nthiebaud@gmail.com>
Date:   Wed Sep 23 02:35:19 2020 -0700

    source 4f2daa1cc683a3e3abbca6e15aa168add414eeff

tdf#102967 remove line at the top of calc header

Change-Id: I9c2cb9d2a87addb42f531a58f29cda351175a39e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103224
Tested-by: Jenkins
Reviewed-by: Andreas Kainz <kainz.a@gmail.com>

Adding CC to: Andreas Kainz

Created attachment 168030 [details]
Calc 7.1 screenshot

Thanks for the bug report Ming Hua, this will show me (again) that for every visual change a bug report is needed.

I remove the horizontal line at the calc header. And the reason was the reduction of visual's in LibO.

If you have a look at calc there is the toolbar (or notebookbar) and in addition the formula bar with the header bar.

For me there is NO need of a horizontal separator between formula bar and header bar. Also the background color of formula bar and headers are the same (see attachment).

Compare to my personal decission there was discussion and research at the design channel about this toppic with the following output:

- there is no reason for a visual separation
- other office suites didn't has this separation
- without the horizontal separation the formula bar isn't locked between separators

So finaly the LibO design group say remove the separator. If a separation is needed background colors are a better solution compare to separator lines.

Created attachment 168038 [details]
Calc 7.1 Beta1 on Windows 10, standard toolbar UI

Hi Andreas,

First, thank you for the detailed explanation.

(In reply to andreas_k from comment #7)
> For me there is NO need of a horizontal separator between formula bar and
> header bar. Also the background color of formula bar and headers are the
> same (see attachment).
I'm also attaching a screenshot, how Calc 7.1 looks with standard toolbar UI and Colibre icon theme, i.e. the default look when it's freshly installed on Windows (I only changed UI language from Chinese to English).

I agree that in your screenshot, with tabbed UI (which have a slightly different BG color) and opened sidebar, the formula bar without seperator line with header bar looks natural.  However I *personally* feel in my screenshot, the missing seperator, especially juxtaposed with the seperator between the standard toolbars and formula bar, is rather visually unpleasent.

> Compare to my personal decission there was discussion and research at the
> design channel about this toppic with the following output:
> 
[...]
> 
> So finaly the LibO design group say remove the separator. If a separation is
> needed background colors are a better solution compare to separator lines.
I of course respect the decisions of developers, especially one after discussion.  So feel free to close this as NOTABUG or WONTFIX.

However I don't think I'm alone, so expect more bugs like this one once 7.1 is released. :-P  Maybe advertise this change a bit more (say, in release notes)?

Thank's for the screenshot. I also think that with standard toolbar it look better without a separator line.

We will see how many bugs will come it's not like I don't respect user feedback, it's good to written down why this change was done.

*** Bug 139603 has been marked as a duplicate of this bug. ***

Created attachment 168879 [details]
Screenshot. MSO2003

I'm siding with comment 0. I'm fine a change -> see MSO look. The vertical need to 'touch' the formula bar, IMHO (without losing the column button proportions)

There is simply something 'off' in the current design

Just to mention that tdf#102967 "remove line at the top of calc header" could not be found here (getting "You must enter a valid bug number!")

*** Bug 141863 has been marked as a duplicate of this bug. ***

Created attachment 171398 [details]
The top border of columns header is missing in Calc 7.1.2.2 repository in Fedora 34 (Workstation Edition Prerelease) x86_64).

I am using Calc 7.1.2.2 repository in Fedora 34 (Workstation Edition Prerelease) x86_64), the top border of columns header is missing. Please bring it back.