#79 Modern: adopt CornerRadius clip-escape workaround, drop status-bar backplate

[scottlerch]

Closes #79.

What

Replaces the modern editor's status-bar backplate hack with the cleaner
CornerRadius workaround proposed on the upstream WinUI issue
(microsoft/microsoft-ui-xaml#11225).

Why

The entries ListView hits a WinUI 3 raster clip-escape bug: once the list's
content extent passes ~2²¹ DIPs, the renderer stops clipping bottom-edge rows and
they paint past the list onto the status bar below (see #72 / #79 for the full
investigation).

The old mitigation was an opaque backplate behind the status bar
(StatusBackplateThreshold / StatusBackplateVisibility + a Border). It only
occluded the strays — rows still rendered out of bounds — and it sacrificed the
true-Mica look on large files by sliding an opaque panel in behind the status text.

Two responders on the upstream issue (lgBlog, duckheadsoftware) independently
reported that giving the ListView any non-zero CornerRadius makes the bug go
away. A non-zero corner radius forces a composition geometry clip on the
ListView's content, which re-establishes the bottom clip — so rows are actually
kept inside the control rather than merely hidden. This works with the transparent
Mica band, so the backplate is no longer needed.

Changes

• Add CornerRadius="0.1" to EntriesList (visually indistinguishable from square).
• Remove StatusBackplateThreshold, StatusBackplateVisibility, the backplate
  Border, and the two OnPropertyChanged(nameof(StatusBackplateVisibility))
  refreshers.
• Update the surrounding comments to document the new approach.

Net: +16 / −44, only MainWindow.xaml / MainWindow.xaml.cs.

Build

dotnet build HostsFileEditor.WinUI (x64, Release + Debug) — 0 warnings, 0 errors.
dotnet format --verify-no-changes — clean.

Manual verification — ✅ confirmed fixed

Verified on a real machine against a ~400K-entry hosts file (loaded via the
HFE_HOSTS_PATH override, well past the ~2²¹-DIP / ~63K-row threshold):

• Bottom-edge rows now clip cleanly — no rows paint over the status bar, at scroll
  offset 0 and mid-list.
• Status band keeps the plain transparent Mica look (no opaque backplate).
• Normal-sized files look identical (the 0.1 radius is imperceptible).

The workaround genuinely fixes the clipping, not just occludes it. The upstream bug is
still open, so the CornerRadius line stays until it ships fixed in a Windows App SDK
(the comment on EntriesList notes this).

🤖 Generated with Claude Code