Auto merge of #17323 - ibmandura:issue16057, r=jdm
Auto merge of #17323 - ibmandura:issue16057, r=jdm

Avoid panic when reloading a page with mouse event handlers

This PR fixes issue #16057, by nullifying `topmost_mouse_over_target` inside of the `handle_exit_pipeline_msg` method, in case owner document is the same as the pipeline document that is being destroyed.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

Testing was done manually, by running `./mach run (-r|-d) "http://localhost:8000/minimal.html" `, where `minimal.html` contains `<div onmouseover="location.reload()" onmouseout="this.clientLeft">hi there</div>`.

r?@jdm

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