profile: Fix glitch when exiting custom profile display UI.

[shubhamdhama]

Fixes: #9702.
GIF:

[zulipbot]

Hello @zulip/server-settings members, this pull request was labeled with the "area: settings (user)" label, so you may want to check it out!

[timabbott]

@shubhamdhama can you talk about how this works and why we should be confident this won't create problems with other modals? E.g. why is 50% correct?

[shubhamdhama]

@timabbott
Three-point we should understand:

• First this applies to hidden modals only because for visible .in containing modal this is overridden.
• We have transform: translateY(-50%); so all the positioning will be done according to the center of modal.
• When we close modal first it moves negative top of the screen and then it hides.

For most of modal top: -25% is more than the whole height of modal but here the modal is so large that the remaining part of modal is visible to us i.e. Height of modal > 25% screen. And this remaining causes glitch. But having set it to -50% is just equivalent to moving it to the same position in a screen of the same size but it negative top(kinda parallel universe :).
This is my understanding and visualization of the issue, hope this makes sense.

[shubhamdhama]

I should add this to commit message if this makes sense.
Oops.. late :)

[timabbott]

OK, that's what I was imagining. Merged as ed7373f, after adding a brief comment explaining that detail. Could also be good to have the longer explanation in the commit message. To provide a bit of reasoning here:

• Anything that answers the question "why does this code do this strange thing" best belongsin a code comment, since that's what'll be most visible to folks reading the code later.
• Anything that explains "why did we make this change" or "what research or reasoning was done to determine this code (change) is correct" generally belongs in the commit message. This is really valuable for folks trying to understand whether changing the code later will break something, but is much more buried than code comments (but also adds much less clutter).

Both are really valuable for saving reviewers time guessing why you did something :)