Voltaire // Disable Rounded Window Corner on macOS 10.11-12.

- macOS 10.11 cannot always render rounded window corner correctly.
- macOS 10.12 users are strongly suggested to upgrade to at least macOS 10.13, especially Hackintosh users for better NVMe support. TBO vChewing is considering of removing macOS 10.12 from the supported OS list.

Source/UI/CandidateUI/HorizontalCandidateController.swift

@@ -105,7 +105,9 @@ fileprivate class HorizontalCandidateView: NSView {
         self.wantsLayer = true
         self.layer?.borderColor = NSColor.selectedMenuItemTextColor.withAlphaComponent(0.30).cgColor
         self.layer?.borderWidth = 1.0
+        if #available(macOS 10.13, *) {
             self.layer?.cornerRadius = 6.0
+        }
         
         let bounds = self.bounds
         NSColor.controlBackgroundColor.setFill() // Candidate list panel base background

Source/UI/CandidateUI/VerticalCandidateController.swift

@@ -111,7 +111,9 @@ fileprivate class VerticalCandidateView: NSView {
         self.wantsLayer = true
         self.layer?.borderColor = NSColor.selectedMenuItemTextColor.withAlphaComponent(0.30).cgColor
         self.layer?.borderWidth = 1.0
+        if #available(macOS 10.13, *) {
             self.layer?.cornerRadius = 6.0
+        }
         
         let bounds = self.bounds
         NSColor.controlBackgroundColor.setFill() // Candidate list panel base background