## Summary of the Pull Request
This PR encompasses the first three bugs we found post-#9820.
### A: Mousedown, select, SCROLL does a weird thing with endpoints that doesn't happen in stable
We were using the terminal position to set the selection anchor, when we should have used the pixel position.
This is fixed in 4f4df01.
### B: Trackpad scrolling down with small increments seems buggy
This one's the most complicated. The touchpad sends very many small scroll deltas, less than one row at a time. The control scrollbar can store a `double`, so small deltas can accumulate. Originally, these would accumulate in the scrollbar, and we'd only read that out as an `int` in the scrollbar updater, which is throttled.
In the interactivity split, there's no place for us to store that double. We immediately narrow to an `int` for `ControlInteractivity::_updateScrollbar`.
So this introduces a double inside `ControlInteractivity` as a fake scrollbar, with which to accumulate to.
This is fixed in 33d29fa...0fefc5b
### C: Looks like there's a selection issue when you click and drag too quickly.
The diff for this one is:
<table>
<tr><td>1.8</td><td>main</td></tr>
<tr>
<td>
```c++
if (_singleClickTouchdownPos)
{
// Figure out if the user's moved a quarter of a cell's smaller axis away from the clickdown point
auto& touchdownPoint{ *_singleClickTouchdownPos };
auto distance{ std::sqrtf(std::powf(cursorPosition.X - touchdownPoint.X, 2) + std::powf(cursorPosition.Y - touchdownPoint.Y, 2)) };
const til::size fontSize{ _actualFont.GetSize() };
const auto fontSizeInDips = fontSize.scale(til::math::rounding, 1.0f / _renderEngine->GetScaling());
if (distance >= (std::min(fontSizeInDips.width(), fontSizeInDips.height()) / 4.f))
{
_terminal->SetSelectionAnchor(_GetTerminalPosition(touchdownPoint));
// stop tracking the touchdown point
_singleClickTouchdownPos = std::nullopt;
}
}
```
</td>
<td>
```c++
if (_singleClickTouchdownPos)
{
// Figure out if the user's moved a quarter of a cell's smaller axis away from the clickdown point
auto& touchdownPoint{ *_singleClickTouchdownPos };
float dx = ::base::saturated_cast<float>(pixelPosition.x() - touchdownPoint.x());
float dy = ::base::saturated_cast<float>(pixelPosition.y() - touchdownPoint.y());
auto distance{ std::sqrtf(std::powf(dx, 2) +
std::powf(dy, 2)) };
const auto fontSizeInDips{ _core->FontSizeInDips() };
if (distance >= (std::min(fontSizeInDips.width(), fontSizeInDips.height()) / 4.f))
{
_core->SetSelectionAnchor(terminalPosition);
// stop tracking the touchdown point
_singleClickTouchdownPos = std::nullopt;
}
}
```
</td>
</tr>
</table>
```c++
_terminal->SetSelectionAnchor(_GetTerminalPosition(touchdownPoint));
```
vs
```c++
_core->SetSelectionAnchor(terminalPosition);
```
We're now using the location of the drag event as the selection anchor, instead of the location that the user initially clicked. Oops.
## PR Checklist
* [x] Checks three boxes, though I'll be shocked if they're the last.
* [x] I work here
* [x] Tests added/passed 🎉🎉🎉
* [n/a] Requires documentation to be updated
## Detailed Description of the Pull Request / Additional comments
All three have tests, 🙌🙌🙌🙌
## Validation Steps Performed
Manual, and automated via tests
c3ca94ceca