Avoid "flashes" while resizing the terminal

We would previously "flash" the screen with the terminal's configured
background color between a call to `Backend::clear` and the following
render. This was because the `Backend::clear` command was not "batched"
in with the render (i.e. synchronized output rendering). We can enable
synchronized outputs in the `Backend::clear` callback to batch the
clear together with the following render to avoid the flash.

Author: the-mikedavis

helix-tui/src/backend/termina.rs

@@ -90,6 +90,7 @@ pub struct TerminaBackend {
     config: Config,
     capabilities: Capabilities,
     reset_cursor_command: String,
+    is_synchronized_output_set: bool,
 }
 
 impl TerminaBackend {
@@ -103,6 +104,7 @@ impl TerminaBackend {
             config,
             capabilities,
             reset_cursor_command,
+            is_synchronized_output_set: false,
         })
     }
 
@@ -297,6 +299,28 @@ impl TerminaBackend {
 
         Ok(())
     }
+
+    // See <https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036>.
+    // Synchronized output sequences tell the terminal when we are "starting to render" and
+    // stopping, enabling to make better choices about when it draws a frame. This avoids all
+    // kinds of ugly visual artifacts like tearing and flashing (i.e. the background color
+    // after clearing the terminal).
+
+    fn start_synchronized_render(&mut self) -> io::Result<()> {
+        if self.capabilities.synchronized_output && !self.is_synchronized_output_set {
+            write!(self.terminal, "{}", decset!(SynchronizedOutput))?;
+            self.is_synchronized_output_set = true;
+        }
+        Ok(())
+    }
+
+    fn end_sychronized_render(&mut self) -> io::Result<()> {
+        if self.is_synchronized_output_set {
+            write!(self.terminal, "{}", decreset!(SynchronizedOutput))?;
+            self.is_synchronized_output_set = false;
+        }
+        Ok(())
+    }
 }
 
 impl Backend for TerminaBackend {
@@ -352,9 +376,7 @@ impl Backend for TerminaBackend {
     where
         I: Iterator<Item = (u16, u16, &'a Cell)>,
     {
-        if self.capabilities.synchronized_output {
-            write!(self.terminal, "{}", decset!(SynchronizedOutput))?;
-        }
+        self.start_synchronized_render()?;
 
         let mut fg = Color::Reset;
         let mut bg = Color::Reset;
@@ -430,9 +452,7 @@ impl Backend for TerminaBackend {
 
         write!(self.terminal, "{}", Csi::Sgr(csi::Sgr::Reset))?;
 
-        if self.capabilities.synchronized_output {
-            write!(self.terminal, "{}", decreset!(SynchronizedOutput))?;
-        }
+        self.end_sychronized_render()?;
 
         Ok(())
     }
@@ -489,6 +509,7 @@ impl Backend for TerminaBackend {
     }
 
     fn clear(&mut self) -> io::Result<()> {
+        self.start_synchronized_render()?;
         write!(
             self.terminal,
             "{}",