[vte] widget: Make scroll adjustment range start at 0 always

[Christian Persch <chpe src gnome org>]

commit cf4984c4c35b6c15b239fbea5abf3a92271665d2
Author: Christian Persch <chpe src gnome org>
Date:   Sat Mar 13 22:10:42 2021 +0100

    widget: Make scroll adjustment range start at 0 always
    
    https://gitlab.gnome.org/GNOME/vte/-/issues/336

 src/vte.cc         | 29 ++++++++++++++++++-----------
 src/vteinternal.hh |  2 +-
 src/widget.cc      | 21 ++++++++++++---------
 3 files changed, 31 insertions(+), 21 deletions(-)
---
diff --git a/src/vte.cc b/src/vte.cc
index fd933e8b..7819dbec 100644
--- a/src/vte.cc
+++ b/src/vte.cc
@@ -7633,27 +7633,34 @@ Terminal::set_size(long columns,
 void
 Terminal::set_scroll_value(double value)
 {
-       /* Save the difference. */
-       auto const dy = value - m_screen->scroll_delta;
+        auto const lower = _vte_ring_delta(m_screen->row_data);
+        auto const upper_minus_row_count = m_screen->insert_delta;
+
+        value = std::clamp(value,
+                           double(lower),
+                           double(std::max(lower, upper_minus_row_count)));
 
-       m_screen->scroll_delta = value;
+        /* Save the difference. */
+        auto const dy = value - m_screen->scroll_delta;
 
-       /* Sanity checks. */
+        m_screen->scroll_delta = value;
+
+        /* Sanity checks. */
         if (G_UNLIKELY(!widget_realized()))
                 return;
 
         /* FIXME: do this check in pixel space */
-       if (!_vte_double_equal(dy, 0)) {
-               _vte_debug_print(VTE_DEBUG_ADJ,
+        if (!_vte_double_equal(dy, 0)) {
+                _vte_debug_print(VTE_DEBUG_ADJ,
                                  "Scrolling by %f\n", dy);
 
                 invalidate_all();
                 match_contents_clear();
-               emit_text_scrolled(dy);
-               queue_contents_changed();
-       } else {
-               _vte_debug_print(VTE_DEBUG_ADJ, "Not scrolling\n");
-       }
+                emit_text_scrolled(dy);
+                queue_contents_changed();
+        } else {
+                _vte_debug_print(VTE_DEBUG_ADJ, "Not scrolling\n");
+        }
 }
 
 Terminal::Terminal(vte::platform::Widget* w,
diff --git a/src/vteinternal.hh b/src/vteinternal.hh
index 1489062e..9e5eabd8 100644
--- a/src/vteinternal.hh
+++ b/src/vteinternal.hh
@@ -438,7 +438,7 @@ public:
                 return _vte_ring_delta (m_screen->row_data);
         }
 
-        inline auto scroll_limit_upper() const noexcept
+        inline constexpr auto scroll_limit_upper() const noexcept
         {
                 return m_screen->insert_delta + m_row_count;
         }
diff --git a/src/widget.cc b/src/widget.cc
index 29b633de..233f36fc 100644
--- a/src/widget.cc
+++ b/src/widget.cc
@@ -918,13 +918,14 @@ Widget::notify_scroll_bounds_changed(bool value_changed)
         auto const freezer = vte::glib::FreezeObjectNotify{m_vadjustment.get()};
         auto changed = false;
 
-        auto dlower = double(terminal()->scroll_limit_lower());
-        auto dupper = double(terminal()->scroll_limit_upper());
+        auto const lower = terminal()->scroll_limit_lower();
+        auto const upper = terminal()->scroll_limit_upper();
+        auto dlower = 0.;
+        auto dupper = double(upper - lower);
         auto dline = 1.;
         auto row_count = terminal()->row_count();
         if (scroll_unit_is_pixels()) [[unlikely]] {
                 auto const factor = m_terminal->get_cell_height();
-                dlower *= factor;
                 dupper *= factor;
                 dline *= factor;
                 row_count *= factor;
@@ -994,9 +995,8 @@ Widget::notify_scroll_value_changed()
         _vte_debug_print(VTE_DEBUG_ADJ,
                          "Updating scroll adjustment value\n");
 
-        m_changing_scroll_position = true;
-
-        auto value = terminal()->scroll_position();
+        auto const lower = terminal()->scroll_limit_lower();
+        auto value = terminal()->scroll_position() - lower;
         if (scroll_unit_is_pixels()) [[unlikely]] {
                 auto const factor = m_terminal->get_cell_height();
                 value *= factor;
@@ -1004,11 +1004,10 @@ Widget::notify_scroll_value_changed()
 
         auto const v = gtk_adjustment_get_value(m_vadjustment.get());
         if (!_vte_double_equal(v, value)) {
-                /* Note that this will generate a 'value-changed' signal */
+                m_changing_scroll_position = true;
                 gtk_adjustment_set_value(m_vadjustment.get(), value);
+                m_changing_scroll_position = false;
         }
-
-        m_changing_scroll_position = false;
 }
 
 #if VTE_GTK == 3
@@ -1594,6 +1593,10 @@ Widget::vadjustment_value_changed()
                 adj /= factor;
         }
 
+        /* Add offset */
+        auto const lower = terminal()->scroll_limit_lower();
+        adj += lower;
+
         m_terminal->set_scroll_value(adj);
 }