Please help me solve the problem and give advice! I will be very grateful!
Question number 1
I made a page for placing a product catalog, which has an upper menu and a side (left) fixed (sticky) menu. The Inline Frame (on the right) displays the corresponding product page (if_1.html, if_2.html, if_1.html). In order for the Inline Frame to display the page without scrolling, I used the iFrame Resizer extension.
Link to the demo project: https://docs.google.com/document/d/1der ... ue&sd=true
On the page "accordion_1.html " by selecting the menu item "SubGroup3 (PROBLEM)" in the "Group1" group in the side menu (of the "accordion" type), a vertical ScrollBar appears in the browser window. If you select an item in the same group, menu item SubGroup1 or SubGroup2, the scrollbar does not appear. Because of this, the elements on the screen are twitching (shifting due to the appearance of the ScrollBar).
Question: how to use the code posted on the page "accordion_1.html " indicate to the browser that the vertical ScrollBar has always been displayed on this page (regardless of whether the page needs to be scrolled or not), which will avoid shifting page elements. Do you have a different solution?
Question number 2
Is there another way (from your development experience) how to make an adaptive page with a fixed vertical menu and display the page corresponding to the selected menu next to it. The displayed pages should be different (when selecting a menu item), and not one with scrolling to the desired anchor. MB. is there a ready-made template with this functionality?
The method I used (in the demo file) required a lot of time and nerves to make it work

Question number 3
Please specify the following question. Bookmarks, where is it more correct to place them, outside the workspace of the page or inside? (The question also applies to extensions, html objects). Sometimes my Bookmarks expanded the page width, and sometimes not. It is very inconvenient to place it inside the page, because it often creates inconveniences in the Layout Grid in terms of the harmonious arrangement of the elements.
Thank you in advance for your help!