Propertly fill window with background color

Close #805

src/terminal.go

@@ -558,6 +558,11 @@ func (t *Terminal) resizeWindows() {
 			width,
 			height, false)
 	}
+	if !t.tui.IsOptimized() && t.theme != nil && t.theme.HasBg() {
+		for i := 0; i < t.window.Height(); i++ {
+			t.window.MoveAndClear(i, 0)
+		}
+	}
 }
 
 func (t *Terminal) move(y int, x int, clear bool) {

src/tui/tui.go

@@ -175,6 +175,10 @@ type ColorTheme struct {
 	Border       Color
 }
 
+func (t *ColorTheme) HasBg() bool {
+	return t.Bg != colDefault
+}
+
 type Event struct {
 	Type       int
 	Char       rune