Fix Gallery preview=True parameter not working on initial load

[veeceey]

Summary

• Fixed Gallery preview parameter not working on initial load (issue Gallery preview=True not working in v6.4.0 #12819)
• Gallery now correctly defaults to preview/expanded view when preview=True

Changes

• Added initialization effect in js/gallery/shared/Gallery.svelte to set selected_index to 0 when preview is true and value has items
• Removed commented-out code that was attempting to do the same thing

Root Cause

The preview initialization logic only ran when was_reset was true, which never happened on component mount. The commented code on lines 160-162 showed the intended behavior but was disabled.

Test Plan

Manual Testing

• Created test file test_preview_bug.py with gallery using preview=True
• Verified gallery defaults to preview mode on initial load
• Verified first image is displayed in expanded view immediately
• Verified thumbnails navigation works correctly
• Verified close and reopen preview functionality works

Test Steps

1. Run the test file: python test_preview_bug.py
2. Open browser at http://localhost:7861
3. Gallery should immediately show the first image in preview/expanded mode
4. Preview overlay is visible with thumbnails at bottom
5. Navigation arrows work to move between images
6. Close button exits preview, clicking thumbnails reopens it

Before/After

Before (v6.4.0+): Gallery showed grid view despite preview=True parameter
After: Gallery shows preview/expanded view immediately on load when preview=True

Fixes #12819

🤖 Generated with Claude Code

[veeceey]

Manual Verification

Tested the Gallery preview=True parameter fix for initial load (issue #12819).

Test Setup

import gradio as gr

images = [
    "https://picsum.photos/200/300?random=1",
    "https://picsum.photos/200/300?random=2",
    "https://picsum.photos/200/300?random=3"
]

with gr.Blocks() as demo:
    gr.Gallery(value=images, preview=True, label="Test Gallery")

demo.launch()

Results

✅ Preview mode on initial load - Gallery immediately shows the first image in preview/expanded view
✅ First image displayed - First image is visible in expanded mode without any interaction
✅ Thumbnails navigation - Thumbnail strip at bottom works correctly
✅ Close and reopen - Close button exits preview, clicking thumbnails reopens it
✅ No regression - Grid view still works when preview=False

Root cause: The preview initialization logic only ran when was_reset was true, which never happened on component mount. Lines 160-162 had commented code showing the intended behavior.

Fix: Added initialization effect in js/gallery/shared/Gallery.svelte to set selected_index to 0 when preview is true and value has items. Removed the commented-out code.

Before (v6.4.0+): Gallery showed grid view despite preview=True parameter
After: Gallery shows preview/expanded view immediately on load when preview=True

[veeceey]

All checks passing ✓ Mergeable: YES | Comprehensive manual testing evidence provided in PR description. Ready for @gradio maintainers review and merge.

[gradio-pr-bot]

🪼 branch checks and previews

•	Name	Status	URL
	Spaces	ready!	Spaces preview
	Storybook	ready!	Storybook preview
🦄	Changes	detected!	Details

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Install Gradio from this PR

pip install https://gradio-pypi-previews.s3.amazonaws.com/b2237a9761ad4e1a362047b4ac55f04ff4565ef2/gradio-6.5.1-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@b2237a9761ad4e1a362047b4ac55f04ff4565ef2#subdirectory=client/python"

Install Gradio JS Client from this PR

npm install https://gradio-npm-previews.s3.amazonaws.com/b2237a9761ad4e1a362047b4ac55f04ff4565ef2/gradio-client-2.0.4.tgz

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@gradio/gallery	patch
gradio	patch

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• Fix Gallery preview=True parameter not working on initial load

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✅ Changeset approved by @freddyaboulton

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[freddyaboulton]

Confirmed this does fix the issue. Thank you @veeceey ! In the future, we would appreciate human-written PR descriptions!