Add support for auto-hiding the sidebar

[Sparker0i]

Summary

Add an option to auto-hide the sidebar so it stays out of the way while browsing. It would slide out when the pointer reaches the window edge and collapse again when the pointer leaves.

Motivation

• The sidebar takes up a fixed amount of horizontal space, and that hurts most on smaller displays and split-screen layouts.
• People who are used to a full-width page view (for example, on docs, notebooks or dashboards) currently have to pick between a permanently visible sidebar and a manual collapse.
• Other vertical-tab browsers offer this, and it's a common request.

Proposed behavior

• Add a setting, off by default so existing behavior doesn't change, to enable "Auto-hide sidebar".
• When enabled, the sidebar is hidden and the page content uses the full window width.
• Moving the pointer to the left (or right, depending on sidebar position) edge of the window reveals the sidebar as an overlay, so the page doesn't reflow or resize.
• The sidebar hides again shortly after the pointer leaves it. A short delay avoids flicker.
• The sidebar stays open while a tab is being dragged, a context menu is open, or an input in it (such as search) has focus.
• Add a keyboard shortcut to toggle the sidebar or reveal it temporarily.

Alternatives considered

• Manually collapsing the sidebar every time. This works but is tedious, and it doesn't give a quick way to peek at tabs.

[EzraCerpac]

I believe this is already the case when the sidebar (or top bar) is collapsed.

[driceroland]

This exists already, off by default: Settings › Tabs › Hide the sidebar until the pointer reaches the edge. The page takes the whole window and the tabs come out when you push the pointer against the left edge; ⌘S keeps them out. A sidebar on the right is planned separately, for later (#253).

[Sparker0i]

One small gripe with the current approach is that if I have my macOS dock on the left and visible, I have to accurately place the cursor on the edge of the left window, otherwise it ends up clicking on the dock.

Maybe something like brave browser could be done here. Show tab icons in the collapsed mode, so that opening the sidebar is much easier.