fix: make Settings app scrollbar draggable

programs/src/settings/main.cpp

@@ -103,6 +103,9 @@ static mtk::TextInputState g_search = {};
 static bool g_search_focus = false;
 static bool g_grid_focus = false;
 static int g_scroll = 0;
+static bool g_sb_hover = false;     // pointer over the scrollbar track
+static bool g_sb_dragging = false;  // dragging the scrollbar thumb
+static int g_sb_drag_offset = 0;    // grab point within the thumb
 static int g_selected = -1;         // index into g_placed (a tile), keyboard focus
 static int g_hover_tile = -1;       // index into g_placed
 static int g_hover_cat = CAT_NONE;
@@ -458,6 +461,18 @@ static void relayout() {
     if (g_hover_tile >= g_placed_count) g_hover_tile = -1;
 }
 
+// The scrollbar spans the whole pane right of the sidebar. Both rects are
+// empty when the content fits.
+static void scrollbar_rects(Rect& track, Rect& thumb) {
+    Rect body = body_rect();
+    track = {0, 0, 0, 0};
+    thumb = {0, 0, 0, 0};
+    if (!mtk::scrollbar_needed(g_content_h, body.h)) return;
+    Rect view = {SIDEBAR_W, 0, g_win.width - SIDEBAR_W, body.h};
+    track = mtk::scrollbar_track_rect(view);
+    thumb = mtk::scrollbar_thumb_rect(track, g_content_h, body.h, g_scroll);
+}
+
 static Rect placed_on_screen(int i) {
     Rect r = g_placed[i].r;
     r.y -= g_scroll;
@@ -681,13 +696,9 @@ static void draw_body(Canvas& c, const mtk::Theme& theme) {
                theme.text_muted);
     }
 
-    Rect body = body_rect();
+    Rect track, thumb;
-    if (mtk::scrollbar_needed(g_content_h, body.h)) {
+    scrollbar_rects(track, thumb);
-        Rect view = {SIDEBAR_W, 0, g_win.width - SIDEBAR_W, body.h};
+    mtk::draw_scrollbar(c, track, thumb, g_sb_hover, g_sb_dragging, theme);
-        Rect track = mtk::scrollbar_track_rect(view);
-        Rect thumb = mtk::scrollbar_thumb_rect(track, g_content_h, body.h, g_scroll);
-        mtk::draw_scrollbar(c, track, thumb, false, false, theme);
-    }
 }
 
 static void draw_header(Canvas& c, const mtk::Theme& theme) {
@@ -883,6 +894,36 @@ static void handle_mouse(const montauk::abi::WinEvent& ev) {
         relayout();
     }
 
+    // Scrollbar: drag the thumb, or click the track to jump there and drag.
+    Rect track, thumb;
+    scrollbar_rects(track, thumb);
+    bool scrollable = !track.empty() && !thumb.empty();
+    if (g_sb_dragging && (!(buttons & 1) || !scrollable)) g_sb_dragging = false;
+    if (g_sb_dragging) {
+        g_scroll = mtk::scrollbar_offset_from_thumb_top(track, g_content_h, body_rect().h,
+                                                        thumb.h, g_mouse_y - g_sb_drag_offset);
+        relayout();
+        g_hover_tile = -1;
+        return;
+    }
+    g_sb_hover = scrollable && track.contains(g_mouse_x, g_mouse_y);
+    if (g_sb_hover) {
+        g_hover_tile = -1;
+        g_hover_cat = CAT_NONE;
+        if (!pressed) return;
+        g_search_focus = false;
+        g_sb_dragging = true;
+        if (thumb.contains(g_mouse_x, g_mouse_y)) {
+            g_sb_drag_offset = g_mouse_y - thumb.y;
+        } else {
+            g_sb_drag_offset = thumb.h / 2;
+            g_scroll = mtk::scrollbar_offset_from_thumb_top(track, g_content_h, body_rect().h,
+                                                            thumb.h, g_mouse_y - g_sb_drag_offset);
+            relayout();
+        }
+        return;
+    }
+
     g_hover_tile = tile_at(g_mouse_x, g_mouse_y);
     g_hover_cat = g_mouse_x < SIDEBAR_W ? sidebar_category_at(g_mouse_x, g_mouse_y) : CAT_NONE;