Do not merge selections on backspace in insert mode

Fixes #2861

src/input_handler.cc

@@ -1235,6 +1235,8 @@ public:
 
             if (not main_char.empty())
                 context().hooks().run_hook(Hook::InsertDelete, main_char, context());
+
+            context().selections_write_only().update(false);
         }
         else if (key == Key::Delete)
         {

src/selection.cc

@@ -218,7 +218,7 @@ void clamp_selections(Vector<Selection>& selections, const Buffer& buffer)
         clamp(sel, buffer);
 }
 
-void update_selections(Vector<Selection>& selections, size_t& main, Buffer& buffer, size_t timestamp)
+void update_selections(Vector<Selection>& selections, size_t& main, Buffer& buffer, size_t timestamp, bool merge)
 {
     if (timestamp == buffer.timestamp())
         return;
@@ -242,6 +242,7 @@ void update_selections(Vector<Selection>& selections, size_t& main, Buffer& buff
         }
         kak_assert(std::is_sorted(selections.begin(), selections.end(),
                                   compare_selections));
+        if (merge)
             selections.erase(
                 merge_overlapping(selections.begin(), selections.end(),
                                   main, overlaps), selections.end());
@@ -249,13 +250,14 @@ void update_selections(Vector<Selection>& selections, size_t& main, Buffer& buff
     for (auto& sel : selections)
         clamp(sel, buffer);
 
+    if (merge)
         selections.erase(merge_overlapping(selections.begin(), selections.end(),
                                            main, overlaps), selections.end());
 }
 
-void SelectionList::update()
+void SelectionList::update(bool merge)
 {
-    update_selections(m_selections, m_main, *m_buffer, m_timestamp);
+    update_selections(m_selections, m_main, *m_buffer, m_timestamp, merge);
     check_invariant();
     m_timestamp = m_buffer->timestamp();
 }

src/selection.hh

@@ -65,7 +65,7 @@ inline bool overlaps(const Selection& lhs, const Selection& rhs)
 }
 
 void update_selections(Vector<Selection>& selections, size_t& main,
-                       Buffer& buffer, size_t timestamp);
+                       Buffer& buffer, size_t timestamp, bool merge = true);
 
 bool compare_selections(const Selection& lhs, const Selection& rhs);
 void sort_selections(Vector<Selection>& selections, size_t& main);
@@ -97,7 +97,7 @@ struct SelectionList
     struct UnsortedTag {};
     SelectionList(UnsortedTag, Buffer& buffer, Vector<Selection> s, size_t timestamp, size_t main);
 
-    void update();
+    void update(bool merge = true);
 
     void check_invariant() const;
 

test/regression/2861-backspace-merges-selections/cmd

@@ -0,0 +1 @@
+a<backspace>oo

test/regression/2861-backspace-merges-selections/in

@@ -0,0 +1 @@
+%(fo)%(fo)%(fo)

test/regression/2861-backspace-merges-selections/out

@@ -0,0 +1 @@
+foofoofoo