fix: Add missing unwatch() method to Callback class

[SuMayaBee]

What's the issue?

While I picked up the issue #7440, the docs for js_on_click say the returned Callback "can be used to disable the callback", so I went ahead and verified that in the docs, then tested it myself:

import panel as pn

button = pn.widgets.Button(name="Open")
cb = button.js_on_click(args=dict(url="google.com"), code="window.open(url, '_blank');")
cb.unwatch()  # AttributeError: 'Callback' object has no attribute 'unwatch'

Confirmed, so the issue persists. The docs doesn't match with the code's output itself.

Root Cause

I went into panel/links.py where the Callback class lives. Here's what I found:

• When a Callback is created via js_on_click, it registers itself into a WeakKeyDictionary called registry — keyed by the source widget.
• _process_callbacks later walks through this registry to attach the JS callbacks to the Bokeh model on render.
• The subclass Link (which extends Callback) already had an unlink() method that simply removes itself from the registry.
• But the base Callback class? Nothing. No unwatch(), no unregister(), no way to undo it at all.

Fix

Added unwatch() to the Callback class, mirroring Link.unlink():

def unwatch(self) -> None:
    source = self.source
    if source is None:
        return
    callbacks = self.registry.get(source, [])
    if self in callbacks:
        callbacks.remove(self)

Notes

• Only prevents the callback from being applied to future renders, consistent with Link.unlink() behaviour.

Closes #7440.

AI Disclosure: I used Gemini 3 Flash to explore and understand the codebase while working on this fix.

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (49872f3) to head (2026b19).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/links.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8453      +/-   ##
==========================================
- Coverage   86.12%   86.01%   -0.11%     
==========================================
  Files         349      349              
  Lines       54971    54996      +25     
==========================================
- Hits        47343    47307      -36     
- Misses       7628     7689      +61     

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.