Detect terminal resize on Windows (fix #4790)

Windows has no SIGWINCH and notifyOnResize was an empty TODO, so fzf
running in --height mode never noticed console size changes. Poll the
console screen buffer dimensions every 100ms and push a signal through
the same channel SIGWINCH uses on Unix. Full-screen mode is unaffected
as it goes through tcell which emits its own resize events.

src/terminal_windows.go

@@ -4,10 +4,48 @@ package fzf
 
 import (
 	"os"
+	"syscall"
+	"time"
+
+	"golang.org/x/sys/windows"
 )
 
+const resizePollInterval = 100 * time.Millisecond
+
+type resizeSignal struct{}
+
+func (resizeSignal) String() string { return "resize" }
+func (resizeSignal) Signal()        {}
+
+// Windows has no SIGWINCH, so poll the console screen buffer for window
+// size changes instead.
 func notifyOnResize(resizeChan chan<- os.Signal) {
-	// TODO
+	consoleOut, err := syscall.Open("CONOUT$", syscall.O_RDWR, 0)
+	if err != nil {
+		return
+	}
+	var info windows.ConsoleScreenBufferInfo
+	if windows.GetConsoleScreenBufferInfo(windows.Handle(consoleOut), &info) != nil {
+		return
+	}
+	last := info.Window
+	go func() {
+		for {
+			time.Sleep(resizePollInterval)
+			if windows.GetConsoleScreenBufferInfo(windows.Handle(consoleOut), &info) != nil {
+				continue
+			}
+			current := info.Window
+			if current.Right-current.Left != last.Right-last.Left ||
+				current.Bottom-current.Top != last.Bottom-last.Top {
+				last = current
+				select {
+				case resizeChan <- resizeSignal{}:
+				default:
+				}
+			}
+		}
+	}()
 }
 
 func notifyStop(p *os.Process) {