Freeze the pane/separator for pinned tab section above vertical tab view panel

[MadhaviSeelam]

Description

Found while testing #29692
After enabling vertical tab view and tabs are pinned, they will show up at the top of the vertical tabs panel. Currently the pane that separates pinned tab section moves when the panel is scrolled up and down and pinned tab section is out of view. Would prefer freezing the pane so pinned tabs are in the view. (Edge has this feature)

Steps to Reproduce

1. Install 1.51.97
2. launch Brave
3. enable vertical tab view
4. add few tabs
5. pin tabs

Actual result:

Pinned sections scrolls up and can't view

2023-04-21_13h04_02.mp4

Expected result:

Freeze the pane for Pinned section

Reproduces how often:

Easily

Brave version (brave://version info)

Brave | 1.51.97 Chromium: 112.0.5615.138 (Official Build) beta (64-bit)
-- | --
Revision | 2258f2fcaeb07bc95c992b7ca47f0b43ead9865c-refs/branch-heads/5615@{#1282}
OS | Windows 11 Version 22H2 (Build 22621.1555)

Version/Channel Information:

• Can you reproduce this issue with the current release? N/A
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

@rebron
cc: @brave/qa-team

[LaurenWags]

++ would like this as well

[stephendonner]

Verified PASSED using

Brave	1.53.75 Chromium: 114.0.5735.58 (Official Build) beta (x86_64)
Revision	ef012197ab4bc867d7b1dd80a001d0be7da8a714-refs/branch-heads/5735_52@{#7}
OS	macOS Version 11.7.7 (Build 20G1345)

Steps:

1. installed 1.53.75
2. launched Brave
3. opened brave://flags
4. switched brave://flags/#brave-vertical-tabs-stick-pinned-tabs to Enabled
5. clicked Relaunch to restart
6. opened a bunch of tabs
7. context-clicked a tab and chose Use Vertical Tabs
8. ensured I had enough tabs for overflow-scrolling
9. pinned a few tabs
10. scrolled
11. unpinned a few tabs
12. dragged and dropped a few pinned tabs' favicons

Confirmed the pinned tabs panel stays as-is (reflecting pinned and unpinned status dynamically), with tabs below scrollable