3950: Display filename of diffed file

dhau****@gmai***** (Google Code) (Is this you? Claim this profile.)
Version: 2.0.18.

When reviewing large changes, one sometimes looses the context and does not remember the current file anymore, which can be confusing.

Possible solution:
Display a small overlay tooltip at the top that shows the current file if no file header is visible in the diff viewer, see attached screenshot. (Additionally, one could add some quick link buttons like "goto next/previous chunk" (currently 'n' and 'p' shortcuts) or similar, if it makes sense.)
#1 david

Merging related tickets.