[BUG] Rounding problem editing numbers on LCD

[drifkind]

Bug Description

When editing some parameters on the LCD, the number loaded into the editor is wrong. (Close, bug not close enough.)

My Configurations

Ender-3 configuration from MarlinConfigurations.

marlin-config.zip

Steps to Reproduce

1. Select Main --> Temperature --> Fan Speed.
2. Set fan speed to (e.g.) 10%.
3. Press knob to re-enter editor. Editor shows 9% instead of 10%.
4. Repeat ad lib.

Also happens some other places, though not quite as easy to find. For example:

1. Select Main --> Configuration --> Advanced --> Temperature --> Fact.
2. Set factor to 0.30. (Or 0.60 or 0.90...)
3. Press knob; editor shows 0.29.

Suggested Fix

--- a/Marlin/src/lcd/menu/menu.h
+++ b/Marlin/src/lcd/menu/menu.h
@@ -236,5 +236,5 @@ class TMenuEditItem : MenuEditItemBase {
     typedef typename NAME::type_t type_t;
     static inline float unscale(const float value)    { return value * (1.0f / NAME::scale);  }
-    static inline float scale(const float value)      { return value * NAME::scale;           }
+    static inline float scale(const float value)      { return roundf(value * NAME::scale);   }
     static void load(void *ptr, const int32_t value)  { *((type_t*)ptr) = unscale(value);     }
     static const char* to_string(const int32_t value) { return NAME::strfunc(unscale(value)); }

[thinkyhead]

Thanks for pointing out the issues. The fan percent issue has been repaired by correcting the rounding. The other items where you see 0.29 instead of 0.30 are due to the issue the way floating point numbers work. Some calculations result in float values that cannot be represented perfectly so you end up seeing a value like 0.2999999. Although the values themselves cannot be rounded to any precision we want, the issue can be addressed by concealing the rounding error on the display, so that's what I have done.

[drifkind]

Ought to mind my own business but...is it possible you overthought that?

[thinkyhead]

We get rounding issues a lot in code world, so we already had FIXFLOAT(x) at the ready.

[github-actions]

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