image: Fix minimal scrollbar with best-fit

For larger images minimal float errors can add up to having
an image size being minimally larger than the widget.
This leads to fake scrollbars that don't even scroll.
Rounding to physical pixel size fixes this.

Fixes #47

src/widgets/image.rs

@@ -1096,33 +1096,41 @@ impl LpImage {
     /// Configure scrollbars for current situation
     fn configure_adjustments(&self) {
         let hadjustment = self.hadjustment();
-        let content_width = self.image_displayed_width();
+        // round to application pixels to avoid tiny rounding errors from zoom
+        let content_width = self.round(self.image_displayed_width());
         let widget_width = self.widget_width();
 
         hadjustment.configure(
             hadjustment.value().clamp(0., self.max_hadjustment_value()),
+            // lower
             0.,
+            // upper
             content_width,
             // arrow button step (probably irrelevant)
             widget_width * 0.1,
             // page up/down step
             widget_width * 0.9,
-            widget_width.min(content_width),
+            // page size
+            f64::min(widget_width, content_width),
         );
 
         let vadjustment = self.vadjustment();
-        let content_height = self.image_displayed_height();
+        // round to application pixels to avoid tiny rounding errors from zoom
+        let content_height = self.round(self.image_displayed_height());
         let widget_height = self.widget_height();
 
         vadjustment.configure(
             vadjustment.value().clamp(0., self.max_vadjustment_value()),
+            // lower
             0.,
+            // upper
             content_height,
             // arrow button step (probably irrelevant)
             widget_height * 0.1,
             // page up/down step
             widget_height * 0.9,
-            widget_height.min(content_height),
+            // page_size
+            f64::min(widget_height, content_height),
         );
     }
 
@@ -1160,4 +1168,12 @@ impl LpImage {
         let list = gdk::FileList::from_array(&[file]);
         Some(gdk::ContentProvider::for_value(&list.to_value()))
     }
+
+    /// Returns scaling aware rounded application pixel
+    ///
+    /// One physical pixel is 0.5 application pixels
+    pub fn round(&self, number: f64) -> f64 {
+        let scale = self.scale_factor() as f64;
+        (number * scale).round() / scale
+    }
 }