megathread: Window State Persistence

[Original thread: #766] [Spec: {none}] [Initial PR: #10972] [Multiple windows PR: #11083]

This thread is being used to track all the component work for restoring the window state. Additionally related threads:

• #766 - Restore the layout, position of windows
  • This is partially implemented in #10972
  • #11083 handles this for multiple windows.
• #961 - Restore the contents of the buffer when resuming a tab.
  • This is not about restoring the process state entirely, since that's impossible. Simply the text content is good enough.
  • Stashing this in JSON doesn't really make sense - imagine SHRT_MAX lines of text + attributes in a json string.
• #960 - Re-open a recently closed tab
  • This means we shouldn't just discard the terminal control when closing a pane/tab. This is similar to the ctrl+shift+t shortcut in (Chromium) browsers
  • Likely best to wait for #1256 and #5000 to sort themselves out before starting this one.
• #8841 and #8812 need to merge, so we can properly reflect all these options in the SUI.
• #11081
• #6362

2.0 Bugs

these are all presuming that #10972 merges basically as is

• We should wrap this feature up in a Preview-only feature flag (no external community member should have to deal with that)
• Opening an elevated window will automatically re-open all the unelevated window state (as elevated!). This is because there are separate monarchs for elevated and unelevated windows.
  • Elevated windows should probably just participate at all in persist/restore for now. That's okay. We may revisit after https://github.com/microsoft/terminal/projects/5#card-67023745
• How do we persist something like wt -- cmd.exe? That won't have a profile, only a NewTerminalArgs, that we don't cache at runtime
  • We've mentioned in team syncs before that we may need to keep the NewTerminalArgs around when spawning a
• Same idea with a defterm connection - We won't be able to restore defterm panes at all, because we have no idea what profile they were. Right?
• How exactly does this work with the _quake window? Should that participate in the session restore? Probably, yea.
• Window names need to be persisted as well.
• Closing the last window with ClosePane will clear out any persisted layout.
  • This is by design
• Similarly with closing the last tab by clicking the x on the tab itself. Doesn't seem to save the window layout in that case.
  • This is by design
• If you have a profile with a background, then the colorscheme will blow away the profile's background on restore.
  • fixed in #11083
• Terminal doesn't remember maximized, focus, fullscreen states #11426
• #11639
• #11878
• #18670
• #12902
• #12633
• The size of a focus mode window is wrong after a session restore. I believe this is actually the same root cause as:
  • #10730
  • as also noted in https://github.com/microsoft/terminal/pull/17066#discussion_r1567425547

This is a tracking megathread rather than a standalone task; start by reading the original thread #766 and the referenced PRs #10972 and #11083. Review the unchecked items and related issues to identify a specific unresolved component, with completion defined by resolving that scoped item rather than the entire window-state persistence effort.