Steps to reproduce:

• Go to the source tab in a repo
• Browse into any file
• Press CTRL+F [The search bar comes up at the top of the file]
• Type something that does occur in the file (do NOT press Enter afterwards)
  [Nothing happens]
• Click down/up arrows on the right side of the search field
  [Nothing happens]

(All this in any browser.)

While writing this, I accidentally found out that if I press enter after typing into the search field, search results actually do appear. (Took me almost a month to discover, most search fields on web pages work differently.)

Clicking the arrows next to the search field (without pressing Enter first) should definitely trigger the search.

Please also consider search-as-you-type (i.e. every character in the search field triggers the search, maybe only after N(3?) characters). As far as I can see, this would not be too resource-intensive, unless the user is browsing a really huge file.