gui: skin_engine: reduce updates

commit ae871d25a9d7781b34e14d3d5c22da2b254ba91a
Author: Christian Soffke <[email protected]>
Date:   Fri May 8 11:32:41 2026 +0200

    gui: skin_engine: reduce updates
    
    Slight optimization of c145d19. Not supposed to result in
    any visible difference.
    
    - if UI viewport is drawn for GUI_EVENT_NEED_UI_UPDATE,
      it doesn't need to do a viewport update; skin_render
      already updates the display
    
    - skin_render_deferred shouldn't need to request that
      the skin perform an immediate update
    
    Change-Id: Id03cf89357eaf0d61af1e928c94942d8c4882dba

diff --git a/apps/gui/skin_engine/skin_render.c b/apps/gui/skin_engine/skin_render.c
index b28a31abc3..eefbc80ee7 100644
--- a/apps/gui/skin_engine/skin_render.c
+++ b/apps/gui/skin_engine/skin_render.c
@@ -850,12 +850,14 @@ void skin_defer_rendering(bool deferred)
 
 void skin_render_deferred(struct screen *display, struct viewport *vp)
 {
+    if (defer_rendering)
+        return;
+
     if (dirty[display->screen_type])
     {
         dirty[display->screen_type] = false;
         display->set_viewport(NULL);
         display->update();
-        sb_skin_force_next_update();
     }
     else
     {
@@ -955,17 +957,18 @@ void skin_render(struct gui_wps *gwps, unsigned refresh_mode)
     skin_backdrop_show(data->backdrop_id);
 #endif
 
+    dirty[display->screen_type] = defer_rendering;
     if (((refresh_mode&SKIN_REFRESH_ALL) == SKIN_REFRESH_ALL))
     {
+        defer_rendering = true;
         /* If this is the UI viewport then let the UI know
          * to redraw itself */
         send_event(GUI_EVENT_NEED_UI_UPDATE, NULL);
+        defer_rendering = dirty[display->screen_type];
     }
     /* Restore the default viewport */
     display->set_viewport_ex(NULL, VP_FLAG_VP_SET_CLEAN);
-    if (defer_rendering)
-        dirty[display->screen_type] = true;
-    else
+    if (!defer_rendering)
         display->update();
 }
 
diff --git a/apps/plugins/lib/simple_viewer.c b/apps/plugins/lib/simple_viewer.c
index bffea2080a..27a30ef0b5 100644
--- a/apps/plugins/lib/simple_viewer.c
+++ b/apps/plugins/lib/simple_viewer.c
@@ -38,6 +38,7 @@ struct view_info {
     int line_count;     /* number of lines */
     int line;           /* current first line */
     int start;          /* possition of first line in text  */
+    bool ui_update_cb;
 };
 
 static bool isbrchr(const unsigned char *str, int len)
@@ -156,6 +157,7 @@ static int init_view(struct view_info *info,
     info->line_count = 0;
     info->line = 0;
     info->start = 0;
+    info->ui_update_cb = false;
 
     if (!info->sbs_has_title)
     {
@@ -223,7 +225,9 @@ static void draw_text(struct view_info *info)
                 info->line_count, info->line, info->line + max_show, VERTICAL);
     }
     display->set_viewport(NULL);
-    display->update();
+    if (!info->ui_update_cb)
+        display->update();
+    info->ui_update_cb = false;
 }
 
 static void scroll_up(struct view_info *info, int n)
@@ -269,6 +273,7 @@ static void ui_update_cb(unsigned short id, void* param, void* user_data)
     (void)id;
     (void)param;
     struct view_info *info = (struct view_info *) user_data;
+    info->ui_update_cb = true;
     draw_text(info);
 }
 
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