fix: prevent DuplicateBlockError in multi-page apps with gr.render

[gambletan]

Summary

• Fixes DuplicateBlockError that occurs when using gr.render in multi-page apps launched via gradio app.py
• The root cause is that Context.id (the global block ID counter) can fall behind the highest block ID in the session's blocks_config after pages are merged, causing newly created blocks in Renderable.apply() to receive IDs that collide with existing blocks
• The fix ensures Context.id is set to at least max(existing_block_ids) + 1 before creating new blocks in the render function

Details

In a multi-page app, when page1.demo.render() and page2.demo.render() are called, blocks from both pages are merged into the root context's blocks_config. When the load event fires and triggers Renderable.apply(), new blocks are created via the user's render function (e.g. gr.Markdown(...)). These blocks get IDs from Context.id, which may not account for all merged block IDs — particularly after a module reload triggered by gradio app.py. This causes the DuplicateBlockError check in Block.render() to fail.

The fix adds a guard at the start of Renderable.apply() that bumps Context.id past the highest existing block ID in the session's config.

Closes #12078

Test plan

• Create a multi-page app with gr.render as described in DuplicateBlockError in multi page app with render #12078
• Run with gradio app.py and verify no DuplicateBlockError on first page load
• Verify the rendered component (e.g. gr.Markdown) displays correctly
• Verify page refresh and code-change reloads still work correctly
• Verify single-page apps with gr.render are unaffected

🤖 Generated with Claude Code

[freddyaboulton]

please do not open duplicate PRs for the same issue.