Interfaces: make edit sidebar resizable

[DarkMatter-999]

What?

Closes #64664

This PR adds the ability to modify the sidebar width either manually or programatically.

Why?

Right now, the editor does not have the ability to change sidebar width, this PR adds support for this.

How?

This PR created a resize-handle which can be dragged using mouse or through keyboard using arrow keys to change the width of the sidebar.

Testing Instructions

1. Open the Block Editor.
2. Grab the left edge of the sidebar
3. Start dragging the edge to change the width and match your preference.

Testing Instructions for Keyboard

1. Tab and reach to the resize-handle (one less tab from the Post and Block buttons bar)
2. Once in focus use arrow keys to modify its width.

Screenshots or screencast

Exposed API to programmatically change sidebar width.

// Get sidebar width.
wp.data.select('core/interface').getComplementaryAreaWidth('core');

// Set sidebar width.
wp.data.dispatch('core/interface').setComplementaryAreaWidth('core', 500);

Screen.Recording.2025-07-04.at.3.01.11.PM.mov

Before

Screen.Recording.2025-07-04.at.3.00.00.PM.mov

After

Screen.Recording.2025-07-04.at.2.59.14.PM.mp4

[github-actions]

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[github-actions]

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[github-actions]

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[yyppsk]

Hey @DarkMatter-999 , I have tested this PR in my local environment and below I have given a detailed report for the outcome.

What: Adds the ability to modify the Block Editor sidebar width manually (drag) and programmatically (API).
Why: The editor previously could not change the sidebar width.
How: Introduces a resize-handle at the sidebar’s left edge; supports mouse drag and keyboard arrow keys.

---

Test Environment

• OS: macOS

• Browsers:

  • Firefox 141.0.3 (aarch64)

  • Chrome latest

  • Safari latest

---

Test Plan & Results

Pointer (Mouse/Trackpad) Resize

Steps

1. Open the Block Editor.

2. Hover the left edge of the sidebar until the resize cursor appears.

3. Drag to adjust the width to preference.

Result

• Width adjusts smoothly.

• No layout breakage observed.

• Consistent behavior across Firefox, Chrome, and Safari on macOS.

API: Programmatic Control

// Get sidebar width
wp.data.select('core/interface').getComplementaryAreaWidth('core');
// Set sidebar width (example: 500px)

wp.data.dispatch('core/interface').setComplementaryAreaWidth('core', 500);

---

---

Observations

• Dragging does not cause visual regressions or layout shifts.

• Behavior appears stable and consistent across tested environments.

• A post-patch video demonstrating the behavior is attached .

---

Video Test :

Screen.Recording.2025-08-17.at.10.16.03.PM.mov

Screenshots

Firefox

Safari

---

Result of my testing

Result: Feature works as expected for pointer resizing with no layout breakage observed.