Alter overflow guard width to avoid text spilling

[oddaaron00]

I've altered the code so that the width of the overflow guard is a few pixels smaller, preventing the text from spilling over the edge of the search box. The Extensions search box is the only place this particular CSS class is used, so this fix won't affect style elsewhere.

This PR fixes #140367

[alexdima]

Unfortunately this is not the correct way to fix this, as the editor can be embedded in many contexts and always subtracting 8 only works in this particular embedding.