brave://welcome jank when resizing (specifically dark mode)

[kjozwiak]

Description

When resizing the brave://welcome page, you'll notice that it's pretty janky/sluggish. Easily noticeable when using the dark theme. Sometimes it will also leave a light border at the bottom as the screen doesn't full repaint when it's resized quickly.

Steps to Reproduce

1. launch 0.69.129 CR: 77.0.3865.90 (make sure you're using dark theme)
2. open brave://welcome and start resizing the window

Actual result:

Expected result:

Resizing brave://welcome shouldn't be this janky/sluggish and should finish repainting the entire window once resized.

Reproduces how often:

100% reproducible when using the above STR.

Brave version (brave://version info)

Brave	0.72.53 Chromium: 77.0.3865.90 (Official Build) nightly (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.14.6 (Build 18G95)

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.14.6 (Build 18G95)
JavaScript	V8 7.7.299.11

Version/Channel Information:

• Can you reproduce this issue with the current release? N/A (hasn't been released yet)
• Can you reproduce this issue with the beta channel? Yes
• Can you reproduce this issue with the dev channel? Yes
• Can you reproduce this issue with the nightly channel? Yes

Other Additional Information:

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

Miscellaneous Information:

CCing @brave/legacy_qa

[btlechowski]

Verification passed on

Brave	0.70.110 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Ubuntu 18.04 LTS

Verified test plan from the description

Verification passed on

Brave	0.70.111 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified test plan from the description

Verified passed with

Brave	0.70.111 Chromium: 77.0.3865.90 (Official Build) beta (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.13.6 (Build 17G5019)

• Verified STR from description