fix scrolling and new row calculation

src/terminal/Screen.zig

@@ -422,7 +422,7 @@ pub fn deinit(self: *Screen) void {
 
 /// Returns true if the viewport is scrolled to the bottom of the screen.
 pub fn viewportIsBottom(self: Screen) bool {
-    return self.viewport >= self.history;
+    return self.viewport == self.history;
 }
 
 /// Shortcut for getRow followed by getCell as a quick way to read a cell.
@@ -577,13 +577,18 @@ fn scrollDelta(self: *Screen, delta: isize, grow: bool) !void {
     self.viewport += @intCast(usize, delta);
     if (self.viewport <= self.history) return;
 
-    // Our viewport is bigger than our max. The number of new rows we need
+    // If our viewport is past the top of our history then we potentially need
-    // in our buffer is our value minus the max.
+    // to write more blank rows. If our viewport is more than our rows written
-    const new_rows_needed = self.viewport - self.history;
+    // then we expand out to there.
+    const rows_written = self.rowsWritten();
+    const viewport_bottom = self.viewport + self.rows;
+    if (viewport_bottom > rows_written) {
+        // The number of new rows we need is the number of rows off our
+        // previous bottom we are growing.
+        const new_rows_needed = viewport_bottom - rows_written;
 
         // If we can't fit into our capacity but we have space, resize the
         // buffer to allocate more scrollback.
-    const rows_written = self.rowsWritten();
         const rows_final = rows_written + new_rows_needed;
         if (rows_final > self.rowsCapacity()) {
             const max_capacity = self.maxCapacity();
@@ -625,6 +630,7 @@ fn scrollDelta(self: *Screen, delta: isize, grow: bool) !void {
             (rows_written_final - 1) * (self.cols + 1),
             self.cols + 1,
         );
+    }
 }
 
 /// Returns the raw text associated with a selection. This will unwrap
@@ -1202,19 +1208,13 @@ test "Screen: scrollback with large delta" {
     }
 
     // Scroll down a ton
-    try s.scroll(.{ .delta = 5 });
+    try s.scroll(.{ .delta_no_grow = 5 });
     try testing.expect(s.viewportIsBottom());
-    {
-        // Test our contents rotated
-        var contents = try s.testString(alloc, .screen);
-        defer alloc.free(contents);
-        try testing.expectEqualStrings("3IJKL\n4ABCD\n5EFGH\n6IJKL", contents);
-    }
     {
         // Test our contents rotated
         var contents = try s.testString(alloc, .viewport);
         defer alloc.free(contents);
-        try testing.expectEqualStrings("6IJKL", contents);
+        try testing.expectEqualStrings("4ABCD\n5EFGH\n6IJKL", contents);
     }
 }
 

src/terminal/point.zig

@@ -44,14 +44,14 @@ pub const Viewport = struct {
         try s.scroll(.{ .delta = 6 });
         try testing.expectEqual(ScreenPoint{
             .x = 0,
-            .y = 6,
+            .y = 3,
         }, (Viewport{ .x = 0, .y = 0 }).toScreen(&s));
 
         // Move the viewport a bit up
         try s.scroll(.{ .delta = -1 });
         try testing.expectEqual(ScreenPoint{
             .x = 0,
-            .y = 5,
+            .y = 2,
         }, (Viewport{ .x = 0, .y = 0 }).toScreen(&s));
 
         // Move the viewport to top