Fix SpinBox value change when held down on separation between buttons

[Giganzo]

Fixes: #110211

Before:

Screencast_20250902_230836.webm

After:

Screencast_20250902_230734.webm

[bruvzg]

Seems to be working as expected, the same code is used for hover/initial press detection.

diff --git a/scene/gui/spin_box.cpp b/scene/gui/spin_box.cpp
index c3632ae4a8..60b23b4a30 100644
--- a/scene/gui/spin_box.cpp
+++ b/scene/gui/spin_box.cpp
@@ -203,7 +203,7 @@ void SpinBox::_range_click_timeout() {
                        double temp_step = get_custom_arrow_step() != 0.0 ? get_custom_arrow_step() : get_step();
                        temp_step = Math::snapped(temp_step, get_step());
                        double new_value = _calc_value(get_value(), temp_step);
-                       if ((mouse_on_up_button && new_value <= get_value() + CMP_EPSILON) || (!mouse_on_up_button && new_value >= get_value() - CMP_EPSILON)) {
+                       if ((mouse_on_up_button && new_value <= get_value() + CMP_EPSILON) || (mouse_on_down_button && new_value >= get_value() - CMP_EPSILON)) {
                                new_value = _calc_value(get_value() + (mouse_on_up_button ? temp_step : -temp_step), temp_step);
                        }
                        set_value(new_value);
@@ -273,7 +273,7 @@ void SpinBox::gui_input(const Ref<InputEvent> &p_event) {
                                        double temp_step = get_custom_arrow_step() != 0.0 ? get_custom_arrow_step() : get_step();
                                        temp_step = Math::snapped(temp_step, get_step());
                                        double new_value = _calc_value(get_value(), temp_step);
-                                       if ((mouse_on_up_button && new_value <= get_value() + CMP_EPSILON) || (!mouse_on_up_button && new_value >= get_value() - CMP_EPSILON)) {
+                                       if ((mouse_on_up_button && new_value <= get_value() + CMP_EPSILON) || (mouse_on_down_button && new_value >= get_value() - CMP_EPSILON)) {
                                                new_value = _calc_value(get_value() + (mouse_on_up_button ? temp_step : -temp_step), temp_step);
                                        }
                                        set_value(new_value);

[Calinou]

Tested locally, it works as expected. Code looks good to me.

Preview with crosshair cursor, so you can see the click area is well-aligned:

spinbox_after.mp4