Steps to reproduce:
# Use slow internet connection (e.g. 300 kbps - east Europe standard) or simulate it
# Find and visit a wiki with banner on the top of every page (currently e.g. cswiki - 2 banners)
# Find and visit any page (best example: any template) on that wiki
# Optional: Click on edit button
Any banner(s), templatedata butons, editor header and other contents of the page heading make the whole edit field (textarea) jumps down after JS is loaded (and therefore these contents of heading are displayed). This effect is even worse on slower internet connections. User then click e.g. in the edit field to write, but it makes nothing or something unwanted, because during or right before his/her click the whole heading loads.
For those contents of page heading there should be reserved a place (space) after start of loading at the beginning of the page in order not to make this jump. The contents then could be displayed after the it is loaded.
0 s: Link to a page was clicked
1 s: The interface is loaded
The edit field (or page text) is loaded
4 s: The heading (banners, templatedata buttons, edit field heading) is loaded and the edit field (or page text) jumps down
PS: This is one of the basic web creator's rule