chore: refactor scroll to bottom logic

[DorraJaouad]

☑️ Resolves

• Refactor scrolling to bottom logic that is used when:
  • Reaction added to the last messages
  • Polling new messages
  • Uploading file
  • Scroll to bottom button

🚧 TODO follow-up :

• Adjust placeholder height to meet clientHeight (it scrolls when joining a conversation) !

[nickvergessen]

Does it still scroll if I have the chat open in a call on my second screen and work in my IDE on the other screen?

[Antreesy]

Filling the truth table for 5 arguments we're using:

🛑 - no scroll at all
❎ - half scroll, user haven't read recent messages
🔽 - to the bottom, user is reading everything
bg - background, tab not visible
fg - foreground, tab visible (on a second screen)
ATB - chat is already at the bottom
⚠️ - was changed with that PR
🚀 - nice to have

Remark: Before has only these rules applied, so behavior might differ a bit:

• force: true - 🔽 upload file only
• isSticky - ATB + messages loaded (not new chat) - 🔽 new messages, new reactions
• isVisible && (hasFocus || isInCall) - 🔽 click to bottom, posting , otherwise ❎ half scroll

Remark: !isVisible && hasFocus is impossible case, omitted from table

visible	focus	InCall	o.force	sticky	before	after	goal	case	diff
0	0	0	0	0	🛑	🛑	🛑	left tab on bg
0	0	0	0	1	❎	❎	❎	left tab ATB
0	0	0	1	0	🔽	❎	🔽	upload and left	⚠️
0	0	0	1	1	🔽	❎	🔽	upload and left ATB	⚠️
0	0	1	0	0	🛑	🛑	🛑	left call on bg
0	0	1	0	1	❎	❎	❎	left call ATB
0	0	1	1	0	🔽	❎	🔽	upload at call bg	⚠️
0	0	1	1	1	🔽	❎	🔽	upload at call bg ATB	⚠️
1	0	0	0	0	🛑	🛑	🛑	chat on fg
1	0	0	0	1	❎	❎	🔽	chat fg, ATB, passive reading	🚀
1	0	0	1	0	🔽	❎	🔽	upload and blur	⚠️
1	0	0	1	1	🔽	❎	🔽	upload ATB and blur	⚠️
1	0	1	0	0	🛑	🛑	🛑	call on fg
1	0	1	0	1	🔽	🔽	🔽	call, passive reading
1	0	1	1	0	🔽	🔽	🔽	upload at call and blur
1	0	1	1	1	🔽	🔽	🔽	upload at call ATB and blur
1	1	0	0	0	🛑	🛑	🛑	reading old messages
1	1	0	0	1	🔽	🔽	🔽	chatting
1	1	0	1	0	🔽	🔽	🔽	upload file
1	1	0	1	1	🔽	🔽	🔽	upload + chat
1	1	1	0	0	🛑	🛑	🛑	reading old chat in call
1	1	1	0	1	🔽	🔽	🔽	chatting in call
1	1	1	1	0	🔽	🔽	🔽	upload in call
1	1	1	1	1	🔽	🔽	🔽	upload + chat in call

if isWindowVisible is falsy, it drops all other cases for you, and when it comes to second if block, you don't have to check for visible/focus/sticky
force should be truthy if you posting a message/file, or at the chat opening, to scroll to the focused / first unread

Looking at the cycle, isInCall and hasFocus can be omitted, and 'force' treated separately before other logic:

if (force) {
    🔽 
} else if (!sticky) {
    🛑 
} else if (!visible) {
    ❎ 
} else {
    🔽
}

[Antreesy]

Algorithm looks fine. Left some comments.

One more thing: loaded chat opens on top, there should be a point or watcher to look after it and set scroll position directly