Hey, good catch @moin. I think you may have found two bugs. 
- The checkbox doesn’t work immediately, and
- Scoping search to a tag creates a route that Discourse serves with a 403
Checkbox isn’t checking
In the mobile the search link adds “context” to the URL, which reveals the checkbox to be toggled in question. In this case, the URL we are looking at is https://meta.discourse.org/search?context=tag&context_id=theme-component&skip_context=true.
When we toggle the checkbox, we are changing the search query where skip_context changes from true to false and back again. It also means you may craft a URL where the box is checked by default, such as https://meta.discourse.org/search?context=category&context_id=1&skip_context=false. This is important, because I’ve noticed different behaviour between the two states when they load.
skip_context=true
The default on mobile view, and the box will not be checked. Interacting with the checkbox, with touch or keyboard, changes the URL to skip_context=false, but does not check the checkbox. Clicking once more will check it, keeping “false” in the URL.
Afterward it may be toggled back and forth, with corresponding URL changes as expected.
skip_context=false
When the search is loaded the checkbox is checked, and interacting with it works as expected: each interaction changes the checkbox and the URL.
I’ve noticed separately that using those checkboxes also causes me to lose focus on the page, regardless of the skip_context value; hopefully it’s the same thing causing our double-click issue.
I’ve reproduced this behavior in safe-mode and on mobile devices.
Okay, I’ll see what I can explain about the tags not scoping in a follow up reply, so it’s easier to split off it needed.