Prevent mouse dragging from dismissing existing selection (#9790)

## PR Checklist
* [x] Closes #9787
* [x] CLA signed.
* [ ] Tests added/passed
* [ ] Documentation updated.
* [ ] Schema updated.
* [ ] I've discussed this with core contributors already. 

## Validation Steps Performed
* [x] single click = no selection
* [x] single click and drag = selection starting from first point
* [x] single click in unfocused pane and drag = focus pane, selection starting from first point
* [x] double-click = selects a whole word
* [x] triple-click = selects a whole line
* [x] double-click and drag = selects a whole word, drag selects whole words
* [x] triple-click and drag = selects a whole line, drag selects whole lines
* [x] Shift single-click = defines start point
* [x] second Shift single-click = defines end point
* [x] Shift double-click = selects entire word
* [x] Shift triple-click = selects entire line
* [x] Shift double-click and drag = selects entire word, drag selects whole words
* [x] Mouse mode: Shift single-click = defines start point
* [x] Mouse mode: second Shift single-click = defines end point
* [x] Mouse mode: Shift double-click = selects entire word
* [x] Mouse mode: Shift triple-click = selects entire line
* [x] Mouse mode: Shift double-click and drag = selects entire word, drag selects whole words
* [x] With existing selection: single-click outside the selection and drag = establishes a new selection starting from the click point 
* [x] Click-drag to set selection, shift-click-drag outside of selection = extend selection while dragging

src/cascadia/TerminalControl/TermControl.cpp

@@ -1422,6 +1422,13 @@ namespace winrt::Microsoft::Terminal::Control::implementation
                     _selectionNeedsToBeCopied = true;
                 }
 
+                if (_terminal->IsSelectionActive())
+                {
+                    // 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;
+                }
+
                 _renderer->TriggerSelection();
             }
             else if (point.Properties().IsRightButtonPressed())