terminal: bound selection drag geometry

Selection drags converted caller-provided floating-point positions directly
to u32 and multiplied geometry dimensions without checking their range.
Non-finite positions, oversized values, overflowing dimensions, or empty
core geometry could therefore panic in runtime safety builds.

Clamp pixel positions to the representable u32 range, reject empty
geometry, and saturate the pixel span when dimensions overflow. Valid
drags keep their existing threshold behavior.

src/terminal/SelectionGesture.zig

@@ -388,8 +388,8 @@ pub fn drag(
         .cell => dragSelection(
             click_pin.*,
             d.pin,
-            @intFromFloat(@max(0, self.left_click_xpos)),
-            @intFromFloat(@max(0, d.xpos)),
+            pixelFromFloat(self.left_click_xpos),
+            pixelFromFloat(d.xpos),
             d.rectangle,
             d.geometry,
         ),
@@ -676,6 +676,19 @@ fn timeSince(time: Time, prev_time: Time) ?u64 {
     return time.since(prev_time);
 }
 
+/// Convert a caller-provided floating-point position to a pixel coordinate.
+/// Negative and NaN values clamp to the origin, matching the drag behavior
+/// outside the left edge of the surface.
+fn pixelFromFloat(value: f64) u32 {
+    if (std.math.isNan(value) or value <= 0) return 0;
+
+    // @intFromFloat requires a value representable by the destination type.
+    // Saturate first so positive infinity and oversized coordinates are safe.
+    const max: f64 = @floatFromInt(std.math.maxInt(u32));
+    if (value >= max) return std.math.maxInt(u32);
+    return @intFromFloat(value);
+}
+
 fn pressSelection(
     self: *const SelectionGesture,
     screen: *Screen,
@@ -720,6 +733,8 @@ fn dragSelection(
     // Rectangular selections are handled similarly, except that
     // entire columns are considered rather than individual cells.
 
+    if (geometry.columns == 0 or geometry.cell_width == 0) return null;
+
     // We only include cells in the selection if the threshold point lies
     // between the start and end points of the selection. A threshold of
     // 60% of the cell width was chosen empirically because it felt good.
@@ -728,7 +743,12 @@ fn dragSelection(
     ));
 
     // We use this to clamp the pixel positions below.
-    const max_x = geometry.columns * geometry.cell_width - 1;
+    const pixel_span = std.math.mul(
+        u32,
+        geometry.columns,
+        geometry.cell_width,
+    ) catch std.math.maxInt(u32);
+    const max_x = pixel_span - 1;
 
     // We need to know how far across in the cell the drag pos is, so
     // we subtract the padding and then take it modulo the cell width.
@@ -1549,6 +1569,58 @@ test "SelectionGesture drag returns selection and records autoscroll" {
     try testing.expectEqual(.down, gesture.left_drag_autoscroll);
 }
 
+test "SelectionGesture drag clamps unrepresentable positions" {
+    var t = try Terminal.init(testing.allocator, .{ .cols = 5, .rows = 5 });
+    defer t.deinit(testing.allocator);
+
+    var gesture: SelectionGesture = .init;
+    defer gesture.deinit(&t);
+
+    var press_event = testPress(&t, 1, 1, try std.time.Instant.now());
+    press_event.xpos = 10;
+    _ = try gesture.press(&t, press_event);
+
+    const positive = gesture.drag(
+        &t,
+        testDrag(&t, 1, 1, std.math.inf(f64), 50),
+    ).?;
+    try testing.expect((testPin(&t, 1, 1)).eql(positive.start()));
+    try testing.expect((testPin(&t, 1, 1)).eql(positive.end()));
+
+    try testing.expectEqual(null, gesture.drag(
+        &t,
+        testDrag(&t, 1, 1, std.math.nan(f64), 50),
+    ));
+}
+
+test "SelectionGesture drag saturates overflowing geometry" {
+    var t = try Terminal.init(testing.allocator, .{ .cols = 5, .rows = 5 });
+    defer t.deinit(testing.allocator);
+
+    var gesture: SelectionGesture = .init;
+    defer gesture.deinit(&t);
+
+    _ = try gesture.press(&t, testPress(&t, 1, 1, try std.time.Instant.now()));
+    var drag_event = testDrag(&t, 1, 1, 10, 50);
+    drag_event.geometry.columns = std.math.maxInt(u32);
+    drag_event.geometry.cell_width = std.math.maxInt(u32);
+    try testing.expectEqual(null, gesture.drag(&t, drag_event));
+}
+
+test "SelectionGesture drag rejects empty geometry" {
+    var t = try Terminal.init(testing.allocator, .{ .cols = 5, .rows = 5 });
+    defer t.deinit(testing.allocator);
+
+    var gesture: SelectionGesture = .init;
+    defer gesture.deinit(&t);
+
+    _ = try gesture.press(&t, testPress(&t, 1, 1, try std.time.Instant.now()));
+    var drag_event = testDrag(&t, 1, 1, 10, 50);
+    drag_event.geometry.columns = 0;
+    drag_event.geometry.cell_width = 0;
+    try testing.expectEqual(null, gesture.drag(&t, drag_event));
+}
+
 test "SelectionGesture release clears autoscroll and records drag" {
     var t = try Terminal.init(testing.allocator, .{ .cols = 5, .rows = 5 });
     defer t.deinit(testing.allocator);