GradientPicker: remove padding and disable overflow of color picker popovers

[ciampo]

What?

As flagged by @t-hamano in #55149 (comment), popovers containing the ColorPicker in the GradientPicker component include some padding around the color picker. This PR removes the padding (and overflow: hidden styles) to have the popover look more uniform with other similar color picker popovers.

Why?

UI polish and consistency

How?

• Remove some ColorPicker-specific styles
• Introduce the resize: false popover prop (which also causes overflow: hidden styles to be applied)
• Use the DropdownContentWrapper component to easily remove the popover's internal padding

Testing Instructions

• Open the GradientPicker Storybook example
• Click on the color stops on the gradient bar
• Notice how the popover containing the gradient picker doesn't have internal padding, and how the drag handle correctly overflows its parent

Screenshots or screencast

Before (trunk)	After (this PR)

[t-hamano]

Thanks for the PR!

It's working as expected, but I'd like to leave a few comments.

[t-hamano]

If this style is removed, an unintended scroll bar will appear when I move the pointer to the right edge of the regular color picker. This problem can be reproduced on Windows, but it might be possible to reproduce it on MacOS by changing the scroll bar settings.

However, strangely, this problem does not occur in #55149, where the same style has been removed.

7eb7c82e55ad7e1c86ebb3999f2e2f63.mp4

[ciampo]

I think that's because it's missing the change to packages/components/src/color-palette/index.tsx setting resize: false for that specific instance of dropdown.

This can be confirmed by removing the resize : false change in this PR in the packages/components/src/custom-gradient-picker/gradient-bar/control-points.tsx file, and noting how the popover in GradientPicker will also start scrolling.

In short, I think this is behaving as expected.

[brookewp]

Looks great! 🚀

[github-actions]

Flaky tests detected in 381fe99.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6493032039
📝 Reported issues:

• [Flaky Test] should select synced pattern #55284 in /test/e2e/specs/editor/various/writing-flow.spec.js