Beside the code for the actual feature this change introduces analysis of the URL to extract a potential filename to be used for saving. Doing so was a prerequisite for disk download because the filename extension in the URL is the only hint for distinguishing ProDOS sector order disk images from DOS 3.3 sector order images.
So far only the URL input was potentially repeated. Now the file input is potentially repeated as well. For normal file paths there's deliberately still no check. But when entering a drive specifier ("!Sx,Dx") then it is checked for plausibility - and there's an additional confirmation question to keep the user from overwriting the "wrong" disk.
With now having both the URL and file input potentially repeated it became clear that loading and saving the input history shouldn't be repeated - and if the user quits without having entered a valid input the history deliberately isn't saved at all.
So far we were reading (much) more bytes from the W5100 than necessary when processing the HTTP header. The byte were memmove'd to the beginning of the buffer.
However, when downloading a DOS 3.3 sector order disk image (*.dsk / *.do) then we need to place every DOS 3.3 sector at a certain point in the buffer to avoid additional memcpy's later. Therefore the HTTP header processing mustn't read (or rather commit) any body bytes from the W5100. So we now just check for the "\r\n\r\n" after each and every byte. This is of course less than optimal but small/simple - and the header isn't supposed to be that large anyway.
IP65 doesn't support TCP flow control. Therefore it doesn't make sense to write a program receiving a significant amount of data via TCP using IP65. From that perspective it makes sense that IP65's HTTP client doesn't allow to handle incoming data with a callback but requires a buffer being able to hold the whole HTTP body.
However, on the Apple II there's the Uthernet II card with its W5100 Ethernet controller chip. That chip has it's own TCP implementation supporting TCP flow control. Therefore the wget65 program uses the W5100 TCP capabilities for the HTTP client.
But even with the W5100 TCP implementation in place IP65 still plays a role for the wget65 program as it desires DHCP and requires (usually) DNS. Both are not supported by the W5100 capabilities.
