From 8140092162f55a50327e2b7dc09bbf08cdef05e6 Mon Sep 17 00:00:00 2001
From: Carlos Zamora <carlos.zamora@microsoft.com>
Date: Tue, 17 Mar 2026 14:46:36 -0700
Subject: [PATCH] Improve UX for selection during VTMM (#19973)
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Selection in VT mouse mode has been feeling a little weird. I've made a
few changes in this space to improve the overall experience when in vt
mouse mode:
- don't round selection positions: this means that you now highlight the
cell you're on if you're going right, and the adjacent cell if you're
going left
- fix drag-left excluding the current cell
- #9608: shift+click now clears any existing selection instead of
extending it. This feels more intuitive since Shift already works as the
override modifier

Somewhat related to #18106

## Validation
✅ alt selections feel consistent
✅ selecting in VTMM feels accurate (selects the cell we started on)
✅ creating new selections (aka clearing old selection) in VTMM feels
intuitive (forwards and backwards)

Closes #9608

(cherry picked from commit 2e33056fd82fcad0bbbe426478d6a686bf612483)
Service-Card-Id: PVTI_lADOAF3p4s4BBcTlzgna3WE
Service-Version: 1.24
---
 src/cascadia/TerminalControl/ControlCore.cpp  |  5 ++++-
 .../TerminalControl/ControlInteractivity.cpp  | 21 ++++++++++++++-----
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/src/cascadia/TerminalControl/ControlCore.cpp b/src/cascadia/TerminalControl/ControlCore.cpp
index fb7d904703..04cc91ca33 100644
--- a/src/cascadia/TerminalControl/ControlCore.cpp
+++ b/src/cascadia/TerminalControl/ControlCore.cpp
@@ -1994,7 +1994,10 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         //   the selection (we need to reset selection on double-click or
         //   triple-click, so it captures the word or the line, rather than
         //   extending the selection)
-        if (_terminal->IsSelectionActive() && (!shiftEnabled || isOnOriginalPosition))
+        // - GH#9608: VT mouse mode is enabled. In this mode, Shift is used
+        //   to override mouse input, so Shift+Click should start a fresh
+        //   selection rather than extending the previous one.
+        if (_terminal->IsSelectionActive() && (!shiftEnabled || isOnOriginalPosition || _terminal->IsTrackingMouseInput()))
         {
             // Reset the selection
             _terminal->ClearSelection();
diff --git a/src/cascadia/TerminalControl/ControlInteractivity.cpp b/src/cascadia/TerminalControl/ControlInteractivity.cpp
index ec249d68f6..37e4bae5e5 100644
--- a/src/cascadia/TerminalControl/ControlInteractivity.cpp
+++ b/src/cascadia/TerminalControl/ControlInteractivity.cpp
@@ -300,8 +300,10 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             }
             const auto isOnOriginalPosition = _lastMouseClickPosNoSelection == pixelPosition;
 
-            // Rounded coordinates for text selection
-            _core->LeftClickOnTerminal(_getTerminalPosition(til::point{ pixelPosition }, true),
+            // Rounded coordinates for text selection.
+            // Don't round in VT mouse mode; cell-level precision matters more
+            const auto round = !_core->IsVtMouseModeEnabled();
+            _core->LeftClickOnTerminal(_getTerminalPosition(til::point{ pixelPosition }, round),
                                        multiClickMapper,
                                        altEnabled,
                                        shiftEnabled,
@@ -311,8 +313,15 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             if (_core->HasSelection())
             {
                 // GH#9787: if selection is active we don't want to track the touchdown position
-                // so that dragging the mouse will extend the selection rather than starting the new one
-                _singleClickTouchdownPos = std::nullopt;
+                // so that dragging the mouse will extend the selection rather than starting the new one.
+                // In VT mouse mode, keep tracking the touchdown point so that PointerMoved
+                // can re-anchor the selection based on drag direction (the dx < 0 adjustment).
+                // Without this, dragging left wouldn't include the initially clicked cell
+                // because floored coordinates place the anchor on the cell's left edge.
+                if (!_core->IsVtMouseModeEnabled())
+                {
+                    _singleClickTouchdownPos = std::nullopt;
+                }
             }
         }
         else if (WI_IsFlagSet(buttonState, MouseButtonState::IsRightButtonDown))
@@ -702,7 +711,9 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     // - cursorPosition: in pixels, relative to the origin of the control
     void ControlInteractivity::SetEndSelectionPoint(const Core::Point pixelPosition)
     {
-        _core->SetEndSelectionPoint(_getTerminalPosition(til::point{ pixelPosition }, true));
+        // Don't round in VT mouse mode; cell-level precision matters more
+        const auto round = !_core->IsVtMouseModeEnabled();
+        _core->SetEndSelectionPoint(_getTerminalPosition(til::point{ pixelPosition }, round));
         _selectionNeedsToBeCopied = true;
     }