Aside:
Did you know you have several icons that don't show up because the icon colour matches your background colour? In your 6th image which shows the topic title being truncated, you can see the image imposed of the TR (it's the 3 lines hamburger menu image). Similarly in your example of the home page, there is a Favourites icon that you can't see. You can hover the mouse over the correct areas and it will change to a hand, indicating the area is clickable. I just thought I'd point it out.
Main topic:
The topic page does seem to behave differently, and that would be something you need to raise with Adobe. You could try support, to see if they have a workaround. Or log a bug report for consideration for a future release.
See the following page for support contact options. The email address is recommended as it goes to a dedicated Robohelp team.
https://helpx.adobe.com/contact/enterprise-support.other.html#robohelp
You can request features and report issues here: https://tracker.adobe.com/
Post the item number in the discussion so people can easily vote if they want the same functionality.
I did poke around the css and I came up with a hack you could try, but Adobe might have a better suggestion. If you want to try it, paste the following into a custom css and add it to the User Assets section of the skin. You might need to add !important after some properties if they don't work once you generate and publish, but I can't test out which ones might be needed.
NOTE: I don't guarantee that this won't cause weird display issues. You'll need to test thoroughly to make sure you're happy with the behaviour on all devices, browsers, screen sizes, etc.
.RH-LAYOUT-HEADER-container {
flex: row wrap;
align-items: unset;
}
.RH-LAYOUT-HEADER-container > * {
flex: 1 100%;
}
.RH-LAYOUT-HEADER-logo-box {
position: unset;
justify-content: unset;
}
.RH-LAYOUT-HEADERMENU-container {
flex: 4 auto;
}
.search-placeholder-class {
display: flex;
flex: 1;
}