widgets: Allow widgets to fill remaining space

This is implemented by propagating the expected remaining space to all
widget draw calls. It is not currently as smart as it could be, but it
works well enough to let the last widget in a row/column fill things.

BarWidget-type widgets and the Launcher widget now fill remaining space
if their length is set to 0.

src/widgets/launcher.rs

@@ -60,7 +60,12 @@ impl<'a> Launcher<'a> {
         })
     }
 
-    fn draw_launcher(&self, buf: &mut Buffer, bg: &Color) -> Result<(), ::std::io::Error> {
+    fn draw_launcher(
+        &self,
+        buf: &mut Buffer,
+        bg: &Color,
+        width: u32,
+    ) -> Result<(), ::std::io::Error> {
         let mut x_off = if !self.input.is_empty() {
             let c = if self.matches.is_empty() {
                 Color::new(1.0, 0.5, 0.5, 1.0)
@@ -77,7 +82,7 @@ impl<'a> Launcher<'a> {
             0
         };
 
-        let mut width_remaining: i32 = (self.length - x_off) as i32;
+        let mut width_remaining: i32 = (width - x_off) as i32;
         for (idx, m) in self.matches.iter().enumerate() {
             let mut b = match buf.offset((x_off, 0)) {
                 Ok(b) => b,
@@ -266,8 +271,12 @@ impl<'a> Widget for Launcher<'a> {
         &mut self,
         ctx: &mut DrawContext,
         pos: (u32, u32),
+        expansion: (u32, u32),
     ) -> Result<DrawReport, ::std::io::Error> {
-        let (width, height) = self.size();
+        if self.length == 0 {
+            self.length = expansion.0;
+        }
+        let (width, height) = (self.length, self.font_size);
         if !self.dirty && !ctx.force {
             return Ok(DrawReport::empty(width, height));
         }
@@ -313,7 +322,7 @@ impl<'a> Widget for Launcher<'a> {
         match self.input.chars().next() {
             Some('=') => self.draw_calc(buf, ctx.bg),
             Some('!') => self.draw_shell(buf, ctx.bg),
-            _ => self.draw_launcher(buf, ctx.bg),
+            _ => self.draw_launcher(buf, ctx.bg, width),
         }?;
 
         Ok(DrawReport {