Is there any way to make the sidebar appear and disappear when I hover in that region?
Yes, basically in the exact same way Arc does.
Having a sidebar always open defeats the purpose, I feel, of being able to maximize screen real estate.
maximize screen real estate and provide a new way to interact with vertical tabs
The sidebar doesn't have to be always open. It can be hidden away either by clicking the "Hide Sidebar" button or CTRL+CMD+S key shortcut.
I agree with this. It’s better to automatically show and hide the sidebar when moving the mouse to the left edge, as in Arc, omitting the mouse click step.
I would actually hate that, so if it is made a thing like in Safari, it should be a setting for sure.
I would like to see there to be an option to enable "show sidebar on hover". Similar to how it is handled in arc. Some improvements that could be made are: give the user an option to select between these modes: minimize to favicons, minimize to specified size, minimize entirely (the sidebar is hidden until a hover event occurs).
Arc has this feature, it saves up space on-screen especially when one needs to have a bazillion tabs opened while also being very easily distracted. This open a topic of splitview with two modes that I will cover in a separate request but still I believe this is the place as these features are somewhat intertwined. Split view is important to my work, I need as much space as possible while also being able to switch between tabs, tab groups, spaces etc. quickly and often visually too. Split view and the sidebar in general should allow for some functionality that I haven't been able to spot in any other browser. I would like to be able to have: split view "pane" be able to not only support a separate sidebar (shares the same data of course, but it is separate meaning that it can have a separate "space" open too (in arc-terms.)) and be able to enable the same window to host multiple tabs from different "windows"/"spaces" with some extendned functionality. These split-view panes should be able to be attached to one another with some logic. For example: there could be a separte sidebar, or sidebar section dedicated to this: "attach this tab-pane to this folder-of-tabs-pane, while leaving the third pane unattached to anything, and name it "my workspace"" "connect sidebars between panes". I will write more about it in a separate request. But it gives you an idea that maybe the splitview should have some things in the sidebar that change the way it works on a ground level. I am not sure. The sidebar should be attached to a pane rather than a window. Also tab folders 
Hey @Vlad, I appreciate your dedication to wanting to build a browser that can feel like it's native. However, I'd still put in a vote for this option. While Edge's implementation may not follow Apple's HIG guildlines, as an end user, I have to say that auto-showing tab names is extremely useful.
This implementation is largely what determines what browser I use. I'm coming from Edge because it's gotten unacceptably slow, sometimes requiring 30 seconds or more just to load a webpage, amongst massive amounts of lag.
I think an option would be amazing. I don't like the double click to expand feature as much. While it might be faster if you're extremely precise about it, it's actually pretty difficult to mouse over that 20 pixels of space and double click it, and then mouse over the new area and click it again—especially if you're quickly trying to find a specific tab and open it.
Both Arc Browser and Zen Browser have a feature where you can have the sidebar automatically hide and show upon hovering the left edge of the browser. This helps free up horizontal space when the sidebar is not in use, and makes vertical tabs far more worthwhile since they do not compromise the amount of available horizontal space you get.
I've made an implementation of this that injects into the app via dylib, but I'd need to disable SIP to use it due to the way WebKit entitlements work.
My implementation adds an NSTrackingArea that covers the entire BrowserWindow object. Whenever the user hovers over the left edge of the tracking area (with about 5px of additional pixels), it calls the private Objective-C _uncollapseWithOverlayArrangedView:animated: method on Orion.SplitView, note that the first argument is the _NSSplitViewItemViewWrapper that contains the sidebar, and the second argument is a BOOL (@(YES) should suffice). This method overlays the sidebar on top of the browser while also animating it. The tracking area checks for when the mouse cursor leaves the sidebar, and when it does it calls _collapseArrangedView:animated: on Orion.SplitView, which, again, takes the _NSSplitViewItemViewWrapper as the first argument, and a boolean as the second.
I do not know what controls the animation however I can note that the animation permanently changes to the existing slide animation after toggling the sidebar with the existing button, so you can likely just call whatever you use to change the animation in that method at BrowserWindow initialization.
These private methods have been in macOS for over 10 years now and I do not expect them to change.
A demo video (not edited, just my implementation) is provided with this post:
This pattern is fairly common in most browsers that have vertical tabs these days, and rightfully so. Tabs are usually not information that needs to be seen at all times, and adding this allows the user to focus on the content more than anything else. I currently do not use vertical tabs in Orion because of how much space it takes up, however adding this would make it my default mode of browsing.
The tricky part with this feature is knowing when to collapse the sidebar. Some apps handle this poorly, where they collapse the sidebar as soon as the mouse leaves the open sidebar. Toonlink’s demo looks good, but doesn’t take into consideration how this works when the window isn’t full-screen.
The way I’ve implement this on websites and custom apps with sidebars is if the pointer passes through the left side of the window and out of the pane, the sidebar opens and doesn’t collapse until the pointer is back in a non-sidebar part of the window. This saves the user from having to carefully keep the pointer inside of the sidebar area to keep it open.
I haven't seen this on the release candidate build yet, unless there is a setting I need to turn on?
Oh wow, is this really going to be implemented?? I can't wait! Hopefully soon on RC!
