refactor(textarea): Improve setting width

When setting the width of the textarea there were some issues
preventing this from working correctly. These problems included:

- If the maximum width needed to be used, the width of the textarea did
  not take into account the prompt and line number width.
- The viewport width did not take into account the style width.

The entire function was confusing to understand and a refactor was
warranted.

As part of this refactor, the bugs mentioned above were fixed and the
code was simplified.

To verify that the logic works as expected, unit tests were expanded to
validate that setting the width works as expected.

Signed-off-by: Michael Lorant <michael.lorant@nine.com.au>

textarea/textarea.go

@@ -862,32 +862,41 @@ func (m *Model) moveToEnd() {
 // It is important that the width of the textarea be exactly the given width
 // and no more.
 func (m *Model) SetWidth(w int) {
+	// Input width must be at least minimum width.
+	inputWidth := max(w, minWidth)
+
+	// Input width must be no more than maximum width.
 	if m.MaxWidth > 0 {
-		m.viewport.Width = clamp(w, minWidth, m.MaxWidth)
-	} else {
-		m.viewport.Width = max(w, minWidth)
+		inputWidth = min(inputWidth, m.MaxWidth)
 	}
 
-	// Since the width of the textarea input is dependent on the width of the
-	// prompt and line numbers, we need to calculate it by subtracting.
-	inputWidth := w
-	if m.ShowLineNumbers {
-		inputWidth -= uniseg.StringWidth(fmt.Sprintf(m.lineNumberFormat, 0))
-	}
+	// Since the width of the viewport and input area is dependent on the width of
+	// borders, prompt and line numbers, we need to calculate it by subtracting
+	// them from the desired width.
 
-	// Account for base style borders and padding.
+	// Subtract base style borders and padding. We need to handle this before we
+	// can set the viewport size.
 	inputWidth -= m.style.Base.GetHorizontalFrameSize()
 
+	// Viewport width can now be set because we known the visible width.
+	m.viewport.Width = inputWidth
+
+	// Subtract line number width from input width.
+	if m.ShowLineNumbers {
+		const lnWidth = 4 // Up to 3 digits for line number plus 1 margin.
+		inputWidth -= lnWidth
+	}
+
+	// Update prompt width only if there is no prompt function as SetPromptFunc
+	// updates the prompt width when it is called.
 	if m.promptFunc == nil {
 		m.promptWidth = uniseg.StringWidth(m.Prompt)
 	}
 
+	// Subtract prompt width from input width.
 	inputWidth -= m.promptWidth
-	if m.MaxWidth > 0 {
-		m.width = clamp(inputWidth, minWidth, m.MaxWidth)
-	} else {
-		m.width = max(inputWidth, minWidth)
-	}
+
+	m.width = inputWidth
 }
 
 // SetPromptFunc supersedes the Prompt field and sets a dynamic prompt