term: scope the conpty resize quirks better to conpty on windows

Previously, we'd just default the resize quirk on for all programs on
Windows.

That pessimizes the otherwise fine behavior of `wezterm ssh` or mux
connections to unixy platforms.

This commit moves the quirk out of the config trait and makes it
a runtime property of the terminal, and then arranges for it
to be set when we know that we set up a conpty for the terminal.

refs: #1265

mux/src/domain.rs

@@ -284,7 +284,7 @@ impl Domain for LocalDomain {
             Box::new(writer),
         );
         if self.is_conpty() {
-            terminal.set_supress_initial_title_change();
+            terminal.enable_conpty_quirks();
         }
 
         let pane: Rc<dyn Pane> = Rc::new(LocalPane::new(

term/src/config.rs

@@ -158,34 +158,6 @@ pub trait TerminalConfiguration: std::fmt::Debug {
     /// defines the initial palette.
     fn color_palette(&self) -> ColorPalette;
 
-    /// Return true if a resize operation should consider rows that have
-    /// made it to scrollback as being immutable.
-    /// When immutable, the resize operation will pad out the screen height
-    /// with additional blank rows and due to implementation details means
-    /// that the user will need to scroll back the scrollbar post-resize
-    /// than they would otherwise.
-    ///
-    /// When mutable, resizing the window taller won't add extra rows;
-    /// instead the resize will tend to have "bottom gravity" meaning that
-    /// making the window taller will reveal more history than in the other
-    /// mode.
-    ///
-    /// mutable is generally speaking a nicer experience.
-    ///
-    /// On Windows, the PTY layer doesn't play well with a mutable scrollback,
-    /// frequently moving the cursor up to high and erasing portions of the
-    /// screen.
-    ///
-    /// This behavior only happens with the windows pty layer; it doesn't
-    /// manifest when using eg: ssh directly to a remote unix system.
-    ///
-    /// Ideally we'd have this return `true` only for the native windows
-    /// pty layer, but for the sake of simplicity, we make this conditional
-    /// on being a windows build.
-    fn resize_preserves_scrollback(&self) -> bool {
-        cfg!(windows)
-    }
-
     fn canonicalize_pasted_newlines(&self) -> NewlineCanon {
         NewlineCanon::default()
     }

term/src/screen.rs

@@ -166,6 +166,7 @@ impl Screen {
         physical_cols: usize,
         cursor: CursorPosition,
         seqno: SequenceNo,
+        is_conpty: bool,
     ) -> CursorPosition {
         let physical_rows = physical_rows.max(1);
         let physical_cols = physical_cols.max(1);
@@ -227,7 +228,29 @@ impl Screen {
 
         let new_cursor_y;
 
-        if self.config.resize_preserves_scrollback() {
+        // true if a resize operation should consider rows that have
+        // made it to scrollback as being immutable.
+        // When immutable, the resize operation will pad out the screen height
+        // with additional blank rows and due to implementation details means
+        // that the user will need to scroll back the scrollbar post-resize
+        // than they would otherwise.
+        //
+        // When mutable, resizing the window taller won't add extra rows;
+        // instead the resize will tend to have "bottom gravity" meaning that
+        // making the window taller will reveal more history than in the other
+        // mode.
+        //
+        // mutable is generally speaking a nicer experience.
+        //
+        // On Windows, the PTY layer doesn't play well with a mutable scrollback,
+        // frequently moving the cursor up to high and erasing portions of the
+        // screen.
+        //
+        // This behavior only happens with the windows pty layer; it doesn't
+        // manifest when using eg: ssh directly to a remote unix system.
+        let resize_preserves_scrollback = is_conpty;
+
+        if resize_preserves_scrollback {
             new_cursor_y = cursor
                 .y
                 .saturating_add(cursor_y as i64)

term/src/terminalstate/mod.rs

@@ -201,13 +201,14 @@ impl ScreenOrAlt {
         cursor_main: CursorPosition,
         cursor_alt: CursorPosition,
         seqno: SequenceNo,
+        is_conpty: bool,
     ) -> (CursorPosition, CursorPosition) {
-        let cursor_main = self
-            .screen
-            .resize(physical_rows, physical_cols, cursor_main, seqno);
-        let cursor_alt = self
-            .alt_screen
-            .resize(physical_rows, physical_cols, cursor_alt, seqno);
+        let cursor_main =
+            self.screen
+                .resize(physical_rows, physical_cols, cursor_main, seqno, is_conpty);
+        let cursor_alt =
+            self.alt_screen
+                .resize(physical_rows, physical_cols, cursor_alt, seqno, is_conpty);
         (cursor_main, cursor_alt)
     }
 
@@ -368,6 +369,7 @@ pub struct TerminalState {
     unicode_version: UnicodeVersion,
     unicode_version_stack: Vec<UnicodeVersionStackEntry>,
 
+    enable_conpty_quirks: bool,
     /// On Windows, the ConPTY layer emits an OSC sequence to
     /// set the title shortly after it starts up.
     /// We don't want that, so we use this flag to remember
@@ -556,6 +558,7 @@ impl TerminalState {
             unicode_version,
             unicode_version_stack: vec![],
             suppress_initial_title_change: false,
+            enable_conpty_quirks: false,
             accumulating_title: None,
             lost_focus_seqno: seqno,
             focused: true,
@@ -564,7 +567,8 @@ impl TerminalState {
         }
     }
 
-    pub fn set_supress_initial_title_change(&mut self) {
+    pub fn enable_conpty_quirks(&mut self) {
+        self.enable_conpty_quirks = true;
         self.suppress_initial_title_change = true;
     }
 
@@ -819,6 +823,7 @@ impl TerminalState {
             cursor_main,
             cursor_alt,
             self.seqno,
+            self.enable_conpty_quirks,
         );
         self.top_and_bottom_margins = 0..physical_rows as i64;
         self.left_and_right_margins = 0..physical_cols;