A pane whose whole text fits in its rows never scrolls to keep a margin

The scroll margin reveals text; with none off screen there is nothing to
reveal. ensureCursorVisible and a centred look both leave such a pane at its
top. A longer file keeps helix's clamp (hxdiff and hxparity unchanged).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 17 insertions, 1 deletions

diff --git a/src/panes.zig b/src/panes.zig
index a7e5c668..47582692 100644
--- a/src/panes.zig
+++ b/src/panes.zig
@@ -512,6 +512,16 @@ pub const Pane = struct {
         }
     }
 
+    /// Every line of the file's text fits in the body's rows; the empty
+    /// line after a final newline is no text.
+    pub fn textFits(pane: *Pane) bool {
+        if (pane.file) |*f| {
+            const lines = File.nlines(pane.gpa, f) - @intFromBool(std.mem.endsWith(u8, f.content, "\n"));
+            return lines <= pane.rows;
+        }
+        return false;
+    }
+
     pub fn ensureCursorVisible(pane: *Pane) void {
         const off = pane.scroll();
         var in_context = false;
@@ -539,7 +549,13 @@ pub const Pane = struct {
                 pane.scrollBy(pane.body.cur_row - margin - off);
             } else if (pane.body.cur_row > last - margin) {
                 var to = pane.body.cur_row + margin;
-                if (pane.file) |*f| to = @min(to, @as(i32, @intCast(File.nlines(pane.gpa, f) -| 1)));
+                if (pane.file) |*f| {
+                    // The margin only reveals text: a pane its whole text
+                    // fits in never scrolls for it. A longer one keeps
+                    // helix's clamp at the buffer's last line.
+                    const lines: i32 = @intCast(File.nlines(pane.gpa, f));
+                    to = @min(to, if (off == 0 and pane.textFits()) @max(pane.body.cur_row, lines - 1 - @intFromBool(std.mem.endsWith(u8, f.content, "\n"))) else lines - 1);
+                }
                 if (comptime Pdf.enabled) {
                     if (pane.pdf) |pv|
                         to = @min(to, @as(i32, @intCast(modal.lineCount(pv.text) -| 1)));