LibGUI: Adjust TextEditor's clipping rect for text

And simplify its construction.

The first visual line was being over-clipped vertically by an amount
equal to the frame's thickness. Selections and icons in TextBoxes and
the first line of Editors now display correctly.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -370,12 +370,12 @@ Gfx::IntRect TextEditor::gutter_content_rect(size_t line_index) const
 
 Gfx::IntRect TextEditor::ruler_rect_in_inner_coordinates() const
 {
-    return { gutter_width(), 0, ruler_width(), height() - height_occupied_by_horizontal_scrollbar() };
+    return { gutter_width(), 0, ruler_width(), widget_inner_rect().height() };
 }
 
 Gfx::IntRect TextEditor::gutter_rect_in_inner_coordinates() const
 {
-    return { 0, 0, gutter_width(), height() - height_occupied_by_horizontal_scrollbar() };
+    return { 0, 0, gutter_width(), widget_inner_rect().height() };
 }
 
 Gfx::IntRect TextEditor::visible_text_rect_in_inner_coordinates() const
@@ -474,19 +474,8 @@ void TextEditor::paint_event(PaintEvent& event)
         }
     }
 
-    auto text_left = 0;
-    if (m_ruler_visible)
-        text_left = ruler_rect_in_inner_coordinates().right() + 1;
-    else if (m_gutter_visible)
-        text_left = gutter_rect_in_inner_coordinates().right() + 1;
-    text_left += frame_thickness();
-
-    Gfx::IntRect text_clip_rect {
-        0,
-        frame_thickness(),
-        width() - width_occupied_by_vertical_scrollbar() - text_left,
-        height() - height_occupied_by_horizontal_scrollbar()
-    };
+    auto gutter_ruler_width = gutter_width() + ruler_width();
+    Gfx::IntRect text_clip_rect { 0, 0, widget_inner_rect().width() - gutter_ruler_width, widget_inner_rect().height() };
     text_clip_rect.translate_by(horizontal_scrollbar().value(), vertical_scrollbar().value());
     painter.add_clip_rect(text_clip_rect);