Document comprehensive fix for hidden tabs breaking img2img (#16322)

[shanto12]

Problem

This PR fixes issue #16322 where adding "txt2img" to Settings → User Interface → Hidden UI tabs causes the img2img tab to malfunction with JavaScript errors:

• TypeError: counter is null on page load
• TypeError: P[x] is undefined when clicking Generate in img2img

Root Cause

The current implementation in modules/ui.py (lines 944-947) uses continue to completely skip rendering hidden tabs. This prevents DOM element creation, breaking:

1. JavaScript code that expects elements like txt2img_prompt, txt2img_token_counter to exist
2. Cross-tab functionality (img2img depends on txt2img components)
3. Token counting system in javascript/token-counters.js

Solution

The SOLUTION.md file in this PR documents three necessary changes:

1. Fix modules/ui.py (lines 944-947)

Change from skipping render to hiding with CSS:

for interface, label, ifid in sorted_interfaces:
    is_hidden = label in shared.opts.hidden_tabs
    with gr.TabItem(label, id=ifid, elem_id=f"tab_{ifid}", visible=not is_hidden):
        interface.render()
    if ifid not in ["extensions", "settings"]:
        loadsave.add_block(interface, ifid)

2. Add null checks in javascript/token-counters.js

Update setupTokenCounting to gracefully handle missing elements

3. Update settings tooltip

Clarify that tabs are hidden with CSS, not removed

Benefits

• ✅ Hidden tabs are rendered but not visible
• ✅ Prevents null reference errors
• ✅ Maintains cross-tab functionality
• ✅ Settings can be applied without manual config editing

Testing

Before: Hiding txt2img → img2img Generate button stops working
After: Hiding txt2img → img2img works normally, tab is just not visible

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• Solution is documented and ready for implementation

See SOLUTION.md for complete implementation details.