Fix scrollbar position calculation

src/Console/Components/Details.php

@@ -14,7 +14,6 @@
 use PhpTui\Tui\Extension\Core\Widget\List\ListItem;
 use PhpTui\Tui\Extension\Core\Widget\ListWidget;
 use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarOrientation;
-use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarState;
 use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarSymbols;
 use PhpTui\Tui\Extension\Core\Widget\ScrollbarWidget;
 use PhpTui\Tui\Style\Style;
@@ -25,6 +24,7 @@
 use Tapper\Console\CommandAttributes\Mouse;
 use Tapper\Console\Component;
 use Tapper\Console\MessageFormatter;
+use Tapper\Console\Support\Scroll;
 
 class Details extends Component
 {
@@ -153,7 +153,7 @@ protected function view(Area $area): Widget
                 ->items(...$allItems)
                 ->offset($this->appState->detailsOffset),
             ScrollbarWidget::default()
-                ->state(new ScrollbarState(max(0, $this->count - $this->area->height), $this->appState->detailsOffset, 1))
+                ->state(Scroll::scrollbarState($this->count, $this->area->height, $this->appState->detailsOffset))
                 ->orientation(ScrollbarOrientation::VerticalRight)
                 ->symbols(new ScrollbarSymbols('│', '█', '', ''))
                 ->endSymbol(null)

src/Console/Components/LogList.php

@@ -11,7 +11,6 @@
 use PhpTui\Tui\Extension\Core\Widget\CompositeWidget;
 use PhpTui\Tui\Extension\Core\Widget\GridWidget;
 use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarOrientation;
-use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarState;
 use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarSymbols;
 use PhpTui\Tui\Extension\Core\Widget\ScrollbarWidget;
 use PhpTui\Tui\Layout\Constraint;
@@ -162,7 +161,7 @@ protected function view(Area $area): Widget
                     ),
                 ),
             ScrollbarWidget::default()
-                ->state(new ScrollbarState(max(0, $this->count - $this->visible), $this->appState->offset, 1))
+                ->state(Scroll::scrollbarState($this->count, $this->visible, $this->appState->offset))
                 ->orientation(ScrollbarOrientation::VerticalRight)
                 ->symbols(new ScrollbarSymbols('│', '█', '', ''))
                 ->endSymbol(null)

src/Console/Support/Scroll.php

@@ -4,12 +4,18 @@
 
 namespace Tapper\Console\Support;
 
+use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarState;
 use Tapper\Console\State\AppState;
 
 class Scroll
 {
     public function __construct(private readonly AppState $appState) {}
 
+    public static function scrollbarState(int $count, int $visible, int $offset): ScrollbarState
+    {
+        return new ScrollbarState(max(0, $count - $visible), $offset, $visible);
+    }
+
     public function cursorDown(int $count, int $visible): void
     {
         if ($this->appState->cursor < $count - 1) {

tests/Unit/ScrollTest.php

@@ -186,3 +186,29 @@
         expect($this->appState->offset)->toBe(5);
     });
 });
+
+describe('scrollbar state', function () {
+    it('maps count/visible/offset onto content length, viewport length and position', function () {
+        $state = Scroll::scrollbarState(count: 100, visible: 20, offset: 35);
+
+        expect($state->contentLength)->toBe(80)
+            ->and($state->viewportContentLength)->toBe(20)
+            ->and($state->position)->toBe(35);
+    });
+
+    it('never lets content length go negative when everything fits on screen', function () {
+        $state = Scroll::scrollbarState(count: 5, visible: 20, offset: 0);
+
+        expect($state->contentLength)->toBe(0);
+    });
+
+    it('uses the max offset (count - visible) as content length so the thumb can reach the bottom', function () {
+        $count = 100;
+        $visible = 20;
+        $maxOffset = $count - $visible;
+
+        $state = Scroll::scrollbarState($count, $visible, $maxOffset);
+
+        expect($state->position)->toBe($state->contentLength);
+    });
+});

tests/Unit/ScrollbarRenderTest.php

@@ -0,0 +1,68 @@
+<?php
+
+declare(strict_types=1);
+
+use PhpTui\Tui\Display\Area;
+use PhpTui\Tui\Display\Buffer;
+use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarOrientation;
+use PhpTui\Tui\Extension\Core\Widget\Scrollbar\ScrollbarSymbols;
+use PhpTui\Tui\Extension\Core\Widget\ScrollbarRenderer;
+use PhpTui\Tui\Extension\Core\Widget\ScrollbarWidget;
+use PhpTui\Tui\Position\Position;
+use PhpTui\Tui\Widget\WidgetRenderer\AggregateWidgetRenderer;
+use Tapper\Console\Support\Scroll;
+
+const THUMB_SYMBOL = '█';
+
+/**
+ * @return string[] one character per row of the rendered scrollbar track
+ */
+function renderScrollbarColumn(int $count, int $visible, int $offset, int $trackHeight): array
+{
+    $area = Area::fromScalars(0, 0, 3, $trackHeight);
+    $buffer = Buffer::empty($area);
+
+    $widget = ScrollbarWidget::default()
+        ->state(Scroll::scrollbarState($count, $visible, $offset))
+        ->orientation(ScrollbarOrientation::VerticalRight)
+        ->symbols(new ScrollbarSymbols('│', THUMB_SYMBOL, '', ''))
+        ->endSymbol(null)
+        ->beginSymbol(null);
+
+    (new ScrollbarRenderer)->render(new AggregateWidgetRenderer([]), $widget, $buffer, $area);
+
+    $column = [];
+    for ($y = 0; $y < $trackHeight; $y++) {
+        $column[] = $buffer->get(Position::at($area->right() - 1, $y))->char;
+    }
+
+    return $column;
+}
+
+it('places the thumb at the very top when scrolled to the top', function () {
+    $column = renderScrollbarColumn(count: 100, visible: 20, offset: 0, trackHeight: 40);
+
+    expect($column[0])->toBe(THUMB_SYMBOL);
+});
+
+it('places the thumb at the very bottom when scrolled to the bottom', function () {
+    $column = renderScrollbarColumn(count: 100, visible: 20, offset: 80, trackHeight: 40);
+
+    expect($column[count($column) - 1])->toBe(THUMB_SYMBOL);
+});
+
+it('sizes the thumb proportionally instead of collapsing to a single row', function () {
+    $column = renderScrollbarColumn(count: 100, visible: 20, offset: 0, trackHeight: 40);
+
+    $thumbRows = count(array_filter($column, fn (string $char) => $char === THUMB_SYMBOL));
+
+    expect($thumbRows)->toBeGreaterThan(1);
+});
+
+it('draws nothing when everything already fits on screen', function () {
+    $column = renderScrollbarColumn(count: 5, visible: 20, offset: 0, trackHeight: 40);
+
+    $thumbRows = count(array_filter($column, fn (string $char) => $char === THUMB_SYMBOL));
+
+    expect($thumbRows)->toBe(0);
+});