Followup to G33/LCD patch #8318

Marlin/ultralcd.cpp

@@ -4746,11 +4746,8 @@ void lcd_update() {
     #endif
 
     if ((lcdDrawUpdate || IS_DRAWING) && (!bbr2 || bbr2 > max_display_update_time)) {
-      #if ENABLED(DOGLCD)
+
-        if (!drawing_screen)
+      if (!IS_DRAWING) switch (lcdDrawUpdate) {
-      #endif
-        {
-          switch (lcdDrawUpdate) {
         case LCDVIEW_CALL_NO_REDRAW:
           lcdDrawUpdate = LCDVIEW_NONE;
           break;
@@ -4761,7 +4758,6 @@ void lcd_update() {
         case LCDVIEW_NONE:
           break;
       } // switch
-        }
 
       #if ENABLED(ADC_KEYPAD)
         buttons_reprapworld_keypad = 0;
@@ -4773,14 +4769,18 @@ void lcd_update() {
         #define CURRENTSCREEN() lcd_status_screen()
       #endif
 
-      #if ENABLED(DOGLCD)  // Changes due to different driver architecture of the DOGM display
+      #if ENABLED(DOGLCD)
-        if (!drawing_screen) {
+        if (!drawing_screen) {                        // If not already drawing pages
-          u8g.firstPage();
+          u8g.firstPage();                            // Start the first page
-          drawing_screen = 1;
+          drawing_screen = 1;                         // Flag as drawing pages
         }
-        lcd_setFont(FONT_MENU);
+        lcd_setFont(FONT_MENU);                       // Setup font for every page draw
-        u8g.setColorIndex(1);
+        u8g.setColorIndex(1);                         // And reset the color
-        CURRENTSCREEN();
+        CURRENTSCREEN();                              // Draw and process the current screen
+
+        // The screen handler can clear drawing_screen for an action that changes the screen.
+        // If still drawing and there's another page, update max-time and return now.
+        // The nextPage will already be set up on the next call.
         if (drawing_screen && (drawing_screen = u8g.nextPage())) {
           NOLESS(max_display_update_time, millis() - ms);
           return;
@@ -4788,6 +4788,9 @@ void lcd_update() {
       #else
         CURRENTSCREEN();
       #endif
+
+      // Keeping track of the longest time for an individual LCD update.
+      // Used to do screen throttling when the planner starts to fill up.
       NOLESS(max_display_update_time, millis() - ms);
     }
 
@@ -4801,11 +4804,7 @@ void lcd_update() {
 
     #endif // ULTIPANEL
 
-    #if ENABLED(DOGLCD)
+    if (!IS_DRAWING) switch (lcdDrawUpdate) {
-      if (!drawing_screen)
-    #endif
-      {
-        switch (lcdDrawUpdate) {
       case LCDVIEW_CLEAR_CALL_REDRAW:
         lcd_implementation_clear();
       case LCDVIEW_CALL_REDRAW_NEXT:
@@ -4817,7 +4816,7 @@ void lcd_update() {
       case LCDVIEW_NONE:
         break;
     } // switch
-      }
+
   } // ELAPSED(ms, next_lcd_update_ms)
 }