Allow starting selections from padding area

[carlos-zamora]

WinUI's Margin and Padding work very similarly. Margin distances
ourselves from our parent. Whereas Padding distances our children from
ourselves.

Terminal's padding setting is actually implemented by defining
Margin on the SwapChainPanel. This means that the "padding" that is
created is actually belongs to SwapChainPanel's parent: Grid (not to be
confused with its parent, "RootGrid").

When a user clicks on the padded area, input goes to Grid. But there's a
twist: you can't actually hit Grid. To be able to hit Grid, you can't
just set IsHitTestVisible. You need to set it's Visibility to Visible,
and it's Background to Transparent (not null) [2].

Validation Steps Performed

• Start a selection from the padding area
• Click on a SearchBox if one is available
  • The SearchBox gets first dibs on the hit test so none gets through
    to the SwapChainPanel

References

[1] https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.uielement.ishittestvisible
[2] https://docs.microsoft.com/en-us/windows/uwp/xaml-platform/events-and-routed-events-overview#hit-testing-and-input-events

Closes #5626

[carlos-zamora]

⚠ This is what's left for me to implement, and why it's a Draft PR. ⚠
Last, we originally "selected" when the pointer drags a quarter of a cell (in TermControl). But we also clamp the converted "cell" to be on the buffer (in TerminalCore). This results in being able to select the first cell while you're still in the padding area. This needs to be fixed such that you have to actually be on the cell you're trying to select.

Marking as ready for review. The issue above just exposes #5099 more. I'm working on that for this milestone anyways.

[DHowett]

I don't understand exactly what's going on here. Can you explain who owns the padding and the margin now? You've removed the SwapChainPanel's margin from calculations, but you're .. still setting it? And the grid that is receiving input doesn't have a margin OR padding?

Can you still click in Search, even when MC is running?

[carlos-zamora]

I don't understand exactly what's going on here. Can you explain who owns the padding and the margin now? You've removed the SwapChainPanel's margin from calculations, but you're .. still setting it? And the grid that is receiving input doesn't have a margin OR padding?

GetPadding() is unchanged. So we still use SwapChainPanel().Margin(). I just replaced a few SwapChainPanel().Margin() with GetPadding() because I'm already here. So, when a user sets a padding, SCP pushes itself away from it's parent (Grid).

When a user interacts with that newly added space, you are not hitting SCP. So, you instead hit the parent (Grid). So I moved the pointer handlers up to the parent. Now, if you click on...

• SCP: it falls through to the parent Grid because there's no handler (this case works, I'm just assuming this is how WinUI implemented it)
• the "padding": you directly hit the Grid
• Search Box: you hit the search box, where the event gets handled and goes no further

The last bit is that to actually pass a hit test, you need to be "visible". So, I made Grid Visible and Transparent.

Can you still click in Search, even when MC is running?

Works as expected :) mouse input is handled on the Search Box and does not leak through to the underlying MC. But if I click on MC while the Search Box is open, focus moves to MC and you actually interact with MC (same as before).

[ghost]

Hello @DHowett!

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[ghost]

🎉Windows Terminal Preview v1.2.2022.0 has been released which incorporates this pull request.:tada:

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