2 files changed, 25 insertions, 4 deletions
diff --git a/src/esp32p4.zig b/src/esp32p4.zig
index 4e1fe036..6772e75d 100644
--- a/src/esp32p4.zig
+++ b/src/esp32p4.zig
@@ -455,8 +455,9 @@ fn apply(c: *pardes.Pardes, ev: vaxis.Event) void {
                         .motion => .motion,
                         .drag => .drag,
                     },
-                    .col = @intCast(m.col),
-                    .row = @intCast(m.row),
+                    // Before the first cell (a drag past the edge): the edge.
+                    .col = @intCast(@max(0, m.col)),
+                    .row = @intCast(@max(0, m.row)),
                     .ctrl = m.mods.ctrl,
                 } });
                 dirty = true;
diff --git a/src/tty/tty.zig b/src/tty/tty.zig
index f0f41747..ca2cb939 100644
--- a/src/tty/tty.zig
+++ b/src/tty/tty.zig
@@ -2466,14 +2466,34 @@ fn mouseEvent(m: vaxis.Mouse) ?pardes.Event {
             .motion => .motion,
             .drag => .drag,
         },
-        .col = @intCast(m.col),
-        .row = @intCast(m.row),
+        .col = cellOf(m.col),
+        .row = cellOf(m.row),
         .ctrl = m.mods.ctrl,
         .shift = m.mods.shift,
         .alt = m.mods.alt,
     } };
 }
 
+/// A report's cell, 0-based: one before the first (SGR's 0, a drag past
+/// the window's top or left edge, in cells or in 1016's pixels) is the
+/// first, never a panic; past the far edge the core clamps it.
+pub fn cellOf(v: i16) u16 {
+    return @intCast(@max(0, v));
+}
+
+test "a mouse report at row or column 0, or before it, is the window's edge, never a panic" {
+    const p = try pardes.Pardes.init(std.testing.allocator, .{ .tty_only = true });
+    defer p.deinit();
+    for ([_][]const u8{ "\x1b[<1;42;0M", "\x1b[<0;0;5M", "\x1b[<32;0;0M", "\x1b[<0;0;0m" }) |sequence| {
+        var parser: vaxis.Parser = .{};
+        const result = try parser.parse(sequence, std.testing.allocator);
+        const m = result.event.?.mouse;
+        const event = mouseEvent(m) orelse continue;
+        try std.testing.expect(event.mouse.col <= 41 and event.mouse.row <= 4);
+        p.update(event);
+    }
+}
+
 test "a mouse's side buttons, xterm's 8 and 9, are Back and Forward, as SDL's X1 and X2 are" {
     for ([_]vaxis.Mouse.Button{ .button_8, .button_9 }, [_]pardes.Mouse.Button{ .back, .forward }) |from, to| {
         const event = mouseEvent(.{ .col = 3, .row = 2, .button = from, .mods = .{}, .type = .press }).?;