Add per-pane title header for split panes (#4717)

[sagarbhure-msft]

Summary of the Pull Request

Adds a per-pane title header bar that appears above each leaf pane when multiple panes are open, allowing users to see all pane titles at a glance without switching focus. Shows a thin title bar above each pane when multiple panes are open. Headers display the pane title, update dynamically via Dispatcher when the shell changes the title (e.g. via escape sequences), and use the focus-state border color as background. Hidden when only one pane exists.

References and Relevant Issues

Closes #4717

Detailed Description of the Pull Request / Additional comments

When two or more panes are visible, a thin header bar appears at the top of each pane displaying its current title. The header updates dynamically when the shell changes the title via escape sequences (e.g. $Host.UI.RawUI.WindowTitle in PowerShell). The focused pane's header uses the focus border color as its background; unfocused panes use the default border color. When a pane is closed and only one remains, headers are automatically hidden.

The header is placed directly in the pane's root Grid as row 0 (auto-sized), with the content border in row 1 (star-sized). This avoids wrapping the TermControl in an extra Grid, which would break SwapChainPanel rendering. A _SetupLeafLayout() helper deduplicates the leaf UI setup between the constructor and the _CloseChild reassembly path.

Validation Steps Performed

• Split panes with Alt+Shift++ (vertical) and Alt+Shift+- (horizontal) — headers appear on all panes
• Set custom titles via $Host.UI.RawUI.WindowTitle — headers update dynamically to show custom titles with emoji
• Clicked between panes — focused pane header highlights blue, others stay dark
• Closed panes until one remains — header disappears automatically
• Tested with up to 6 simultaneous panes — all headers render correctly with proper layout
• Verified terminal content renders without transparency or artifacts after splittin

Screenshots

Screenshots (6 panes, focus cycling through each)

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PR Checklist

• Closes Add title to split window #4717
• Tests added/passed
• Documentation updated
  • If checked, please file a pull request on our docs repo and link it here: #xxx
• Schema updated (if necessary)

[carlos-zamora]

Wow! Thanks for doing this! The demo photos look great and it doesn't look like a complex diff.

There's a few notes I want to discuss:

• Would it be possible for this to be a setting?
  • Most of the boilerplate gets added in via the macros here: https://github.com/microsoft/terminal/blob/main/src/cascadia/TerminalSettingsModel/MTSMSettings.h
  • In the TerminalSettingsEditor, it would best go in https://github.com/microsoft/terminal/blob/main/src/cascadia/TerminalSettingsEditor/GlobalAppearance.xaml
  • Something simple like "displayPaneHeaders" should work, imo
• Accessibility:
  • I'm curious if the headers are read out by screen readers automatically? They're just set up to be the title of the content, so I suspect that it'll be a very clean and accessible experience, but I want to double check.

Aside from that, I think this is pretty much ready to go. I'm adding Needs-Discussion just to touch base with the team on it since it's a decently big UI change, but I'm excited to see this land!