HexEditor: Clarify code for selecting byte colors

Instead of assigning, then sometimes reassigning these colors, set them
once. This makes it easier to see how we prioritize the different
factors that affect the styling.

`highlight_flag` is now `selected` since it represents if the byte is
within the current selection.

Author: AtkinsSJ

Userland/Applications/HexEditor/HexEditor.cpp

@@ -634,10 +634,6 @@ void HexEditor::paint_event(GUI::PaintEvent& event)
 
             auto const cell = m_document->get(byte_position);
 
-            bool const selection_inbetween_start_end = byte_position >= m_selection_start && byte_position < m_selection_end;
-            bool const selection_inbetween_end_start = byte_position >= m_selection_end && byte_position < m_selection_start;
-            bool const highlight_flag = selection_inbetween_start_end || selection_inbetween_end_start;
-
             Gfx::IntRect hex_display_rect_high_nibble {
                 frame_thickness() + offset_margin_width() + j * cell_width() + 2 * m_padding,
                 frame_thickness() + m_padding + i * line_height(),
@@ -663,27 +659,39 @@ void HexEditor::paint_event(GUI::PaintEvent& event)
             auto high_nibble = line.substring_from_byte_offset(0, 1).release_value_but_fixme_should_propagate_errors();
             auto low_nibble = line.substring_from_byte_offset(1).release_value_but_fixme_should_propagate_errors();
 
-            Gfx::Color background_color_hex = palette().color(background_role());
-            Gfx::Color background_color_text = background_color_hex;
-            Gfx::Color text_color_hex = [&]() -> Gfx::Color {
+            bool const selected = min(m_selection_start, m_selection_end) <= byte_position
+                && byte_position < max(m_selection_start, m_selection_end);
+
+            // Styling priorities are as follows, with smaller numbers beating larger ones:
+            // 1. Modified bytes
+            // 2. The cursor position
+            // 3. The selection
+            // 4. Null bytes
+            // 5. Regular formatting
+            auto determine_background_color = [&](EditMode edit_mode) -> Gfx::Color {
+                if (selected)
+                    return cell.modified ? palette().selection().inverted() : palette().selection();
+                if (byte_position == m_position && m_edit_mode != edit_mode)
+                    return palette().inactive_selection();
+                return palette().color(background_role());
+            };
+            auto determine_text_color = [&](EditMode edit_mode) -> Gfx::Color {
                 if (cell.modified)
                     return Color::Red;
+                if (selected)
+                    return palette().selection_text();
+                if (byte_position == m_position)
+                    return (m_edit_mode == edit_mode) ? palette().color(foreground_role()) : palette().inactive_selection_text();
                 if (cell.value == 0x00)
                     return palette().color(ColorRole::PlaceholderText);
                 return palette().color(foreground_role());
-            }();
-            Gfx::Color text_color_text = text_color_hex;
+            };
+            Gfx::Color background_color_hex = determine_background_color(EditMode::Hex);
+            Gfx::Color background_color_text = determine_background_color(EditMode::Text);
+            Gfx::Color text_color_hex = determine_text_color(EditMode::Hex);
+            Gfx::Color text_color_text = determine_text_color(EditMode::Text);
             auto& font = cell.modified ? this->font().bold_variant() : this->font();
 
-            if (highlight_flag) {
-                background_color_hex = background_color_text = cell.modified ? palette().selection().inverted() : palette().selection();
-                text_color_hex = text_color_text = cell.modified ? text_color_hex : palette().selection_text();
-            } else if (byte_position == m_position) {
-                background_color_hex = (m_edit_mode == EditMode::Hex) ? background_color_hex : palette().inactive_selection();
-                text_color_hex = (m_edit_mode == EditMode::Hex) ? text_color_text : palette().inactive_selection_text();
-                background_color_text = (m_edit_mode == EditMode::Text) ? background_color_text : palette().inactive_selection();
-                text_color_text = (m_edit_mode == EditMode::Text) ? text_color_text : palette().inactive_selection_text();
-            }
             painter.fill_rect(background_rect, background_color_hex);
 
             Gfx::IntRect text_display_rect {