Rotary encoder on status screen can change flow percentage

Marlin/Configuration_adv.h

@@ -1474,6 +1474,7 @@
   #if IS_ULTIPANEL
     #define MANUAL_E_MOVES_RELATIVE // Display extruder move distance rather than "position"
     #define ULTIPANEL_FEEDMULTIPLY  // Encoder sets the feedrate multiplier on the Status Screen
+    //#define ULTIPANEL_FLOWPERCENT // Encoder sets the flow percentage on the Status Screen
   #endif
 #endif
 

Marlin/src/inc/Conditionals_LCD.h

@@ -972,6 +972,7 @@
   #define DETECT_I2C_LCD_DEVICE 1
 #endif
 
+// Encoder behavior
 #ifndef STD_ENCODER_PULSES_PER_STEP
   #if ENABLED(TOUCH_SCREEN)
     #define STD_ENCODER_PULSES_PER_STEP 2

Marlin/src/inc/SanityCheck.h

@@ -4147,6 +4147,10 @@ static_assert(WITHIN(MULTISTEPPING_LIMIT, 1, 128) && IS_POWER_OF_2(MULTISTEPPING
   #endif
 #endif
 
+#if ALL(ULTIPANEL_FEEDMULTIPLY, ULTIPANEL_FLOWPERCENT)
+  #error "Only enable ULTIPANEL_FEEDMULTIPLY or ULTIPANEL_FLOWPERCENT, but not both."
+#endif
+
 // Misc. Cleanup
 #undef _TEST_PWM
 #undef _NUM_AXES_STR

Marlin/src/lcd/dogm/status_screen_DOGM.cpp

@@ -888,7 +888,12 @@ void MarlinUI::draw_status_screen() {
     lcd_put_lchar(3, EXTRAS_2_BASELINE, LCD_STR_FEEDRATE[0]);
 
     set_font(FONT_STATUSMENU);
-    lcd_put_u8str(12, EXTRAS_2_BASELINE, i16tostr3rj(feedrate_percentage));
+
+    #if ENABLED(ULTIPANEL_FLOWPERCENT)
+      lcd_put_u8str(12, EXTRAS_2_BASELINE, i16tostr3rj(planner.flow_percentage[active_extruder]));
+    #else
+      lcd_put_u8str(12, EXTRAS_2_BASELINE, i16tostr3rj(feedrate_percentage));
+    #endif
     lcd_put_u8str(F("%"));
 
     //

Marlin/src/lcd/marlinui.cpp

@@ -700,7 +700,7 @@ void MarlinUI::init() {
         else
           new_frm = old_frm;
       }
-      else if ((old_frm < 100 && new_frm > 100) || (old_frm > 100 && new_frm < 100))
+      else if ((old_frm < 100) == (new_frm > 100)) // Crossing 100? Stick at 100.
         new_frm = 100;
 
       LIMIT(new_frm, SPEED_EDIT_MIN, SPEED_EDIT_MAX);
@@ -720,7 +720,31 @@ void MarlinUI::init() {
         #endif
       }
 
-    #endif // ULTIPANEL_FEEDMULTIPLY
+    #elif ENABLED(ULTIPANEL_FLOWPERCENT)
+
+      const int16_t old_fp = planner.flow_percentage[active_extruder];
+            int16_t new_fp = old_fp + int16_t(encoderPosition);
+
+      // Dead zone at 100% flow percentage
+      if (old_fp == 100) {
+        if (int16_t(encoderPosition) > ENCODER_FEEDRATE_DEADZONE)
+          new_fp -= ENCODER_FEEDRATE_DEADZONE;
+        else if (int16_t(encoderPosition) < -(ENCODER_FEEDRATE_DEADZONE))
+          new_fp += ENCODER_FEEDRATE_DEADZONE;
+        else
+          new_fp = old_fp;
+      }
+      else if ((old_fp < 100) == (new_fp > 100)) // Crossing 100? Stick at 100.
+        new_fp = 100;
+
+      LIMIT(new_fp, FLOW_EDIT_MIN, FLOW_EDIT_MAX);
+
+      if (old_fp != new_fp) {
+        planner.set_flow(active_extruder, new_fp);
+        encoderPosition = 0;
+      }
+
+    #endif // ULTIPANEL_FLOWPERCENT
 
     draw_status_screen();
   }