image/zoom: Allow 50% zoom via menu for 16px images

Zoom out for 16px or smaller images is usually blocked. But allow it for
zoom menu as we allow 50% via zoom menu in other situations where it is
blocked otherwise. Also, this is probably much less complex
implementation wise.

Closes #544

news.d/50.alpha/changed-limit-zoom-out

@@ -0,0 +1 @@
+Don't allow endless zoom out

src/widgets/image.rs

@@ -130,6 +130,9 @@ const SMALL_OCCUPY_SCREEN: f64 = 0.8;
 /// this value, unless the image itself is smaller than this.
 const SMALLEST_RENDERED_PIXEL_SIZE: f64 = 16.0;
 
+/// Smallest zoom level reachable via entries in the zoom menu
+const LOWEST_MENU_ZOOM_LEVEL: f64 = 0.5;
+
 #[derive(Default, Debug, Clone, Copy, glib::Variant, glib::Enum, PartialEq, Eq)]
 #[enum_type(name = "LpFitMode")]
 pub enum FitMode {

src/widgets/image/zoom.rs

@@ -99,20 +99,25 @@ impl imp::LpImage {
     pub(super) fn min_zoom(&self) -> f64 {
         let (width, height) = self.untransformed_dimensions();
 
-        // If the image is smaller than the smallest rendered pixel size,
+        let zoom_level = if (width as f64) < SMALLEST_RENDERED_PIXEL_SIZE
-        // block zooming out entirely
-        if (width as f64) < SMALLEST_RENDERED_PIXEL_SIZE
             || (height as f64) < SMALLEST_RENDERED_PIXEL_SIZE
         {
+            // If the image is smaller than the smallest rendered pixel size,
+            // block zooming out entirely
             1.0
-        }
+        } else if width < height {
             // Prevent the rendered size from going below SMALLEST_RENDERED_PIXEL_SIZE
-        // Original size * zoom factor = rendered size
-        // <=> Zoom factor = rendered size / original size
-        else if width < height {
             SMALLEST_RENDERED_PIXEL_SIZE / (width as f64)
         } else {
             SMALLEST_RENDERED_PIXEL_SIZE / (height as f64)
+        };
+
+        if matches!(self.obj().fit_mode(), FitMode::ExactVolatile) {
+            // Allow zoom menu to allways function. We are allowing this since disabling the
+            // zoom menu entries would probably be more complicated.
+            f64::min(zoom_level, LOWEST_MENU_ZOOM_LEVEL)
+        } else {
+            zoom_level
         }
     }