It does not fully answer the question, but I think I would still gain valuable knowledge about the codebase indirectly, rather than being utterly slow trying to navigate a big codebase on my phone.
Grep the function calls, perhaps doctrings too
Call back the person you foolishly told to expect an immediate appraisal and remark 'I have investigated that old codebase.'
Set up an appointment to discuss it at length and then browse it more comfortably over the weekend on a normal screen instead of torturing your corneas
As for myself, I need glasses and have thick fingers, so I would even think of trying.