feat: replace WebView2+xterm.js with native Microsoft.Terminal.Wpf

[sailro]

Replace WebView2+xterm.js with native Microsoft.Terminal.Wpf

Summary

Replace the Chromium-based terminal renderer (WebView2 + xterm.js) with VS's built-in native terminal control (Microsoft.Terminal.Wpf) — the same DirectWrite rendering engine used by Windows Terminal and VS's own integrated terminal.

Motivation

The embedded terminal (added in PR #7) used WebView2 to host xterm.js. While functional, this carried significant overhead:

	WebView2 + xterm.js (before)	Microsoft.Terminal.Wpf (after)
Renderer	Chromium Canvas/WebGL	Native Win32 DirectWrite
VT parsing	JavaScript (xterm.js)	Native C++ (Windows Terminal engine)
Memory	~80MB (Chromium process)	~5MB (in-process WPF control)
Startup	Slow (Chromium init + page load)	Instant (WPF control)
Dependencies	WebView2 runtime + 6 JS files	None (ships with VS)
Focus	Complex WPF↔Chromium recovery hack	Native WPF focus model
Theme	Hardcoded dark theme in JS	VS theme integration via SetTheme()
Box-drawing	Needed WebGL addon workaround	Perfect natively

How it works

Microsoft.Terminal.Wpf.dll ships with every Visual Studio installation at CommonExtensions\Microsoft\Terminal\. Our extension references it at build time and resolves it at runtime via an AssemblyResolve handler that locates the DLL from the VS install directory.

The new TerminalToolWindowControl implements ITerminalConnection — a 4-member interface:

• WriteInput(string data) — user keystroke → ConPTY
• TerminalOutput event — ConPTY output → native renderer
• Resize(uint rows, uint columns) — dimensions changed
• Close() — cleanup

The ConPTY layer (TerminalProcess, TerminalSessionService) is completely unchanged — same I/O pipeline, just a different consumer.

Changes

New files

• TerminalThemer.cs (79 lines) — VS theme → TerminalTheme with dark/light ANSI palettes (COLORREF/BGR format), auto-switches on VSColorTheme.ThemeChanged
• TerminalToolWindowControl.cs — rewritten as ITerminalConnection implementation (183 lines, down from 209)

Removed files

• Resources/Terminal/terminal.html — WebView2 host page
• Resources/Terminal/terminal-app.js — xterm.js bridge, resize, I/O
• Resources/Terminal/lib/xterm.js — xterm.js core
• Resources/Terminal/lib/xterm.css — xterm.js styles
• Resources/Terminal/lib/addon-fit.js — FitAddon
• Resources/Terminal/lib/addon-webgl.js — WebglAddon

Modified files

• CopilotCliIde.csproj — removed WebView2 NuGet + Terminal resources, added Terminal.Wpf assembly reference
• CopilotCliIdePackage.cs — added static AssemblyResolve handler for Microsoft.Terminal.Wpf
• Directory.Packages.props — removed Microsoft.Web.WebView2 package version
• TerminalToolWindow.cs — simplified (removed WebView2-specific PreProcessMessage)
• TerminalSessionService.cs — removed stale comment
• package-lock.json — cleaned ghost xterm references

Documentation updated

• .github/copilot-instructions.md — architecture now references Microsoft.Terminal.Wpf
• README.md — terminal stack updated
• CHANGELOG.md — migration documented under [Unreleased]

Unchanged (ConPTY layer)

• TerminalProcess.cs — ConPTY process management
• TerminalSessionService.cs — session lifecycle
• ConPty.cs — P/Invoke bindings
• All MCP server files — terminal is client-side only

Stats

16 files changed, 317 insertions(+), 639 deletions(-)

Net removal of 322 lines — simpler codebase with better rendering.

Testing

• Build clean (0 errors, 0 warnings)
• 284 server tests pass
• Roslyn dead code analysis clean (1 pre-existing unused constant)
• Manual testing: Copilot CLI renders with full ANSI colors, box-drawing characters, status bar, interactive prompt
• Theme integration: dark background from VS ToolWindowBackgroundColorKey
• Session restart (Enter after process exit) works
• Resize handling (dock panel drag) works
• VS 2022 compatibility (built/tested on VS 2026 Insiders only)
• Light theme testing

Notes

• Microsoft.Terminal.Wpf.dll is not bundled in the VSIX — it resolves from the VS install directory at runtime. This avoids redistribution concerns and ensures version compatibility with the host VS.
• The AssemblyResolve handler in the package's static constructor finds devenv.exe's directory via Process.GetCurrentProcess().MainModule.FileName, then looks for the DLL at CommonExtensions\Microsoft\Terminal\.
• ITerminalConnection.Resize may be called from a non-UI thread by the native control. The session start (which needs GetWorkspaceFolder() on the UI thread) is marshaled via Dispatcher.BeginInvoke.

[sailro]

Hey @bommerts, thank you again for the initial prototype. I tried to move to the native terminal support that we have in VS, and it worked. I need more testing of course, but it could be great.

[bommerts]

Pulled the branch, built clean, all 284 tests pass. This is a huge improvement over the WebView2 approach. Some observations from reading through the code:

What's great:

• The ITerminalConnection interface is really elegant. The control IS the connection instead of needing a separate JS bridge layer. Much cleaner.
• e.Handled = true in OnGotFocus (line 116) is exactly the fix for the infinite focus loop we fought with WebView2. Native WPF focus just works.
• No more deferred/lazy WebView2 init, no Chromium process, no cache directory. The constructor is 18 lines instead of 300+.
• OnSessionRestarted re-syncing ConPTY dimensions is a nice touch that we missed in the WebView2 version.
• Keeping OnUnloaded as a no-op (session survives hide/show) is the right call.

Minor observations:

1. ITerminalConnection.Resize() can be called from a non-UI thread by the native control. The StartSession path correctly marshals via Dispatcher.BeginInvoke, but the else branch (line 89) calls _sessionService?.Resize() directly on that thread. This works because TerminalProcess.Resize() uses a lock internally, but it might be worth a comment noting the thread safety assumption.
2. In the AssemblyResolve handler, if neither path exists it silently returns null. Might be helpful to log a warning so users on unusual VS installs get a diagnostic hint instead of a confusing load failure later.

I'm going to do hands-on testing in VS 2026 Enterprise and will report back with results.

[bommerts]

Tested the native-terminal branch in VS 2026 Enterprise. Everything works: terminal renders, typing is responsive, process restart on Enter works, resize works. Looks identical to the WebView2 version from a user perspective, which is the point. Nice work!

One minor UX thing I noticed (pre-existing, not from this PR): clicking Tools > Copilot CLI Window doesn't auto-focus the terminal. I'll open a separate issue for that after this merges.

[sailro]

We should be good now, I addressed observations