[atorresg]

What

Adds Settings ▸ General ▸ Windows ▸ "Open each document in its own window".

QuickMD deliberately puts every document in the frontmost window as a tab
(WindowTabbing.preferTabsAlways() plus the manual merge in
joinExistingTabs), so two documents can never be read side by side. With
the new option on, each document gets its own window instead.

Off by default — the tab behaviour is unchanged.

Why

Reading two documents at once (a spec and the code that implements it, two
related notes) is a normal thing to want, and the tab layout makes it
impossible without a second app instance.

How

Three AppKit levers, with different lifetimes — the distinction is the
whole difficulty here:

Lever	Scope	When it takes effect
NSWindow.userTabbingPreference (AppleWindowTabbingMode in the argument domain)	global	At launch only
NSWindow.allowsAutomaticWindowTabbing	global	any time
NSWindow.tabbingMode	per window	when the window is shown

• At launch, the preference is written as "manual" instead of "always",
  and allowsAutomaticWindowTabbing is set to false. The preference has
  to be chosen here: AppKit reads userTabbingPreference once per process
  and caches it, so rewriting the defaults domain later has no effect.
• While the app runs, the two levers that do work at runtime are used:
  allowsAutomaticWindowTabbing (global) and tabbingMode (per window).
• The manual merge that joins a new window to an existing tab group is
  skipped in separate-windows mode.

Note for the record: the raw value for "never tab" is "manual", not
"never" — "never" is an old alias for "inFullScreen".

Limitation

Documents that are already open keep the tab group they are in. AppKit
gives an app no way to take a tab group apart (addTabbedWindow(_:ordered:)
is public, removeTabbedWindow is not), so the option applies to documents
opened from the moment it is switched on. At launch it applies to
everything.

Tests

WindowTabbingTests covers the preference values written to the argument
domain, allowsAutomaticWindowTabbing in both modes, the per-window mode,
that the manual merge is skipped, and that switching the option back and
forth restores the previous behaviour.

Checked by hand as well: with the option on, every document is its own
window (window.tabbedWindows is nil for each); with it off, they all
join one tab group.

[b451c]

Thank you, this is in 1.15.0: Settings > General > "Open documents in separate windows", off by default.

Changes on top of your branch:

• with the option on QuickMD uses the same mode as the system's "Prefer tabs: Never", so Merge All Windows and Move Tab to New Window stay available when it is on from launch;
• new windows cascade and remember the last size;
• the caption says that documents already open stay as they are and that it takes full effect after a restart, for the reason you described (the tabbing preference is read once per process).

You are credited in the changelog.