Is it possible to stop toolbars (&/or sidebars) from moving web pages around?
In fullscreen mode with "hide toolbars" checked, it would be nice if web content didn't get shifted to make way for the tab bar/navigation bar. This would avoid the following problem: toolbars shown, the webpage shifts down, toolbars hidden, the webpage shifts up, and so on. The toolbars should be "overlaid" instead.
Is it possible to change this via userChrome.css?
The same thing also happens when using sidebars (although the top toolbars are my main concern). Can they be overlaid rather than pushing things aside?
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The "sliding" of page content up and down as the user enters and exits Full Screen mode has always been "the way it works". There have been a couple of preferences in Firefox over the years that allowed a slight bit of modification of that action, but only to the extent that the "sliding" could be immediate or it could be slow movement - the default action currently.
Yes, it would be nice if page content stayed in place, bit that's not what the developers want. Over the years there have been a number of "Full Screen" add-ons to change that action, but iirc the last one available was probably available for Firefox 28. And the add-on that I used for many years - Autohide - was last available for Firefox 3.6, with only an incomplete "glitchy" beta version working for Firefox 4.0; by Firefox 10 is was kaput.
I guess we'll have to wait and see if an add-on developer can build a Web Extension version of that for the future; or if Mozilla provides an API where an extension like that is possible.
As far as the sidebar goes; that action differs from that of the Full Screen feature. When the sidebar is open the page / browser window content is compressed, not slid off the screen to the right or left; depending upon how the user has specified where they want their sidebar to open.
I don't know if it is possible to "overlay" the browser window content or not. There have been changes to the sidebar placing more emphasis on the sidebar than I have ever seen with Firefox. For many years my feeling is that the developers treated the sidebar as a relic of the 90's; got "little to no love". But in the last year or so that seems to have changed, almost to the point that it is getting too much attention, and in the process "screwing up" how it works for those of us who have always used the sidebar; for me it has been my "go to" for accessing my bookmarks and it has gotten "clunkier" recently, IMO.
As far as a change to being overlaid - IMO doubtful; but we'll have to see if an API to do that becomes available for a Web Extension developer to make use of. IMO, if it can be done with Chrome (and all the other WebKit/Blink browsers out there), Firefox will get an API to make that happen. And if no one else has the feature, Firefox may or may not get it.