fix: prevent infinite resize loop when footer_links=[] on HuggingFace Spaces

[Ker102]

Summary

Fixes #12992 — fill_height=True combined with footer_links=[] causes the interface to grow infinitely on HuggingFace Spaces.

Root Cause

handle_resize() in Blocks.svelte only runs when "parentIFrame" in window (HuggingFace Spaces iframe environment). It calculates box.bottom + footer_height + 32 and calls window.parentIFrame.size(new_height) to tell HF to resize the iframe.

This creates a feedback loop:

1. ResizeObserver fires → calls handle_resize()
2. handle_resize() reports height → HF resizes the iframe
3. Iframe grows → box.bottom increases → ResizeObserver fires again
4. New height is bigger → reports again → infinite loop

When the footer exists, footer_height is bound to the footer's clientHeight via bind:clientHeight={footer_height}. The footer acts as a stable height anchor — once content + footer height settles, the resize stabilizes. But with footer_links=[], the footer is removed from the DOM entirely ({#if footer_links.length > 0}), footer_height stays at 0, and there's nothing to break the feedback loop.

This explains the behavior reported in #12992:

• Works locally — no parentIFrame, so handle_resize() is a no-op
• Works embedded on custom sites — is_embed=true sets fill_height=false
• Only breaks on HuggingFace Spaces — parentIFrame exists and drives the resize

Fix

When fill_height is active and there is no footer (footer_height === 0), skip the parentIFrame.size() call entirely.

This is semantically correct — fill_height means "fill the available space in the parent container", so asking the parent to resize to match the content height is contradictory and creates the feedback loop.

Testing

Verified with a simulation test covering 4 scenarios:

• ✅ fill_height=true, footer_links=[] → resize loop prevented (0 resize cycles)
• ✅ fill_height=true, footer present → resize works normally
• ✅ fill_height=false, no footer → resize works normally
• ✅ fill_height=false, footer present → completely unaffected

[pngwn]

Don’t delete the template.