Add middle-button scrolling support

[ghnew]

Summary of the Pull Request

Introduces Mouse Panning (middle-button scrolling) and a configurable middleClickAction setting to Windows Terminal (wt.exe).

Users can now middle-click to enter vertical autoscroll panning mode with dynamic cursor icon feedback (SizeAll ╬ at the origin anchor and SizeNorthSouth $\updownarrow$ while moving), or configure middle-click to paste text (common in Unix/Linux terminals).

References and Relevant Issues

• Closes Mouse panning (middle-button scrolling) #1701

Detailed Description of the Pull Request / Additional comments

1. Settings Model (src/cascadia/TerminalSettingsModel/ & src/cascadia/inc/):

   • Added MiddleClickAction enum (Default = 0, Pan, Paste, None) and MiddleClickAction property to IControlSettings.idl.
   • Added macro definition entry to CONTROL_SETTINGS(X) in ControlProperties.h.
   • Exposed middleClickAction in profile settings (Profile.idl, MTSMSettings.h).
   • Added JSON string mapper definitions in TerminalSettingsSerializationHelpers.h and EnumMappings.cpp to support "pan", "paste", "default", and "none" values in settings.json.

2. Input Interactivity & Viewport Control (src/cascadia/TerminalControl/):

   • ControlInteractivity.cpp: Updated PointerPressed for IsMiddleButtonDown to evaluate MiddleClickAction. When set to Paste, it raises RequestPasteTextFromClipboard().
   • TermControl.cpp:
     • Added _isPanning and _panningAnchorPos state tracking in _PointerPressedHandler, _PointerMovedHandler, and _PointerReleasedHandler.
     • Calculates vertical displacement velocity relative to the origin touchdown point and feeds it to _TryStartAutoScroll(...).
     • Dynamically sets CoreWindow::GetForCurrentThread().PointerCursor(...) to CoreCursorType::SizeAll ╬ when stationary near the anchor point and CoreCursorType::SizeNorthSouth $\updownarrow$ while scrolling vertically. Resets the cursor to IBeam when panning completes.

3. Unit Testing (src/cascadia/UnitTests_Control/):

   • Added TestMiddleClick to ControlInteractivityTests.cpp to test settings evaluation and clipboard paste event dispatching.

Validation Steps Performed

• Automated Tests: Executed TAEF unit tests via te.exe on Control.Unit.Tests.dll. Verified TestMiddleClick passed (1/1 Passed).
• Manual Verification:
  1. Built and launched WindowsTerminal.exe locally.
  2. Verified middle-clicking inside the terminal viewport enters autoscroll panning mode with cursor switching (SizeAll $\rightarrow$ SizeNorthSouth).
  3. Added "middleClickAction": "paste" to settings.json and verified middle-clicking pastes clipboard text.
  4. Verified middle-clicks pass through to applications (e.g. vim / tmux) when VT mouse tracking is active.

PR Checklist

• Closes Mouse panning (middle-button scrolling) #1701
• Tests added/passed
• Documentation updated
• Schema updated (if necessary)

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[ghnew]

@microsoft-github-policy-service agree

[DHowett]

Mind sharing a video?

[ghnew]

Screen.Recording.2026-07-27.111029.mp4

@DHowett sure, here it is

[lhecker]

The PR looks good overall, but this should technically be an action / keybinding. Since we don't have proper mouse-bindings yet, could we just mark this setting as experimental like some others?

[ghnew]

Thanks for the review @lhecker! That makes total sense. I've updated the setting JSON key to "experimental.middleClickAction" in MTSMSettings.h.

[lhecker]

I don't think we need a default value, right? The default is unambiguously scrolling, at least on Windows.

[ghnew]

I've removed the redundant "default" string option and made Pan (0) the default enum value for experimental.middleClickAction so the JSON options are cleanly "pan", "paste", and "none".

[DHowett]

Hey, so, can somebody clarify who @SanjayPJ is compared to @ghnew, and whether @ghnew has permission to submit @SanjayPJ's commit and sign the contributor license agreement for @SanjayPJ's work?

[ghnew]

Hi @DHowett ! Sorry for the confusion.

Both @SanjayPJ and @ghnew are my GitHub accounts. I accidentally used different accounts for the contribution and the CLA.

To keep everything consistent, I've now rewritten the commit author so that all commits are attributed to @ghnew, which is the account associated with the CLA. Please let me know if there's anything else I need to do.

[lhecker]

Do note that GitHub's TOS technically forbids having multiple non-paid accounts.