Viewer: Fix zoom step size setting ignored. Fixes #1875840 [Viewer's font "zoom step size" preference dysfunctional](https://bugs.launchpad.net/calibre/+bug/1875840)

src/calibre/gui2/viewer/web_view.py

@@ -549,12 +549,6 @@ class WebView(RestartingWebEngineView):
         if ans is not None and not sip.isdeleted(ans):
             return ans
 
-    def change_zoom_by(self, steps=1):
-        # TODO: Add UI for this
-        ss = vprefs['session_data'].get('zoom_step_size') or 20
-        amt = (ss / 100) * steps
-        self._page.setZoomFactor(max(0.25, min(self._page.zoomFactor() + amt, 5)))
-
     def render_process_died(self):
         if self.dead_renderer_error_shown:
             return

src/pyj/read_book/prefs/font_size.pyj

@@ -37,9 +37,10 @@ def set_custom_size(ev):
     sz = int(element(CONTAINER, 'input').value)
     change_font_size(sz)
 
-def change_font_size_by(amt):
+def change_font_size_by(frac):
     sd = get_session_data()
     sz = sd.get('base_font_size')
+    amt = sz * frac
     nsz = sz + amt
     nsz = max(8, min(nsz, 80))
     change_font_size(nsz)

src/pyj/read_book/prefs/fonts.pyj

@@ -16,6 +16,11 @@ DEFAULT_MINIMUM_FONT_SIZE = 8
 DEFAULT_ZOOM_STEP_SIZE = 20
 
 
+def current_zoom_step_size():
+    s = get_session_data().get('standalone_font_settings')
+    return s.zoom_step_size or DEFAULT_ZOOM_STEP_SIZE
+
+
 def font_select(name, settings):
     ans = E.select(name=name)
     ans.appendChild(E.option(_('— Choose a font —'), value=''))

src/pyj/read_book/view.pyj

@@ -13,7 +13,9 @@ from dom import add_extra_css, build_rule, clear, set_css, svgicon, unique_id
 from iframe_comm import IframeWrapper
 from modals import error_dialog, warning_dialog
 from read_book.content_popup import ContentPopupOverlay
-from read_book.create_annotation import AnnotationsManager, CreateAnnotation, ViewAnnotation
+from read_book.create_annotation import (
+    AnnotationsManager, CreateAnnotation, ViewAnnotation
+)
 from read_book.globals import (
     current_book, runtime, set_current_spine_item, ui_operations
 )
@@ -22,6 +24,7 @@ from read_book.open_book import add_book_to_recently_viewed
 from read_book.overlay import Overlay
 from read_book.prefs.colors import resolve_color_scheme
 from read_book.prefs.font_size import change_font_size_by
+from read_book.prefs.fonts import current_zoom_step_size
 from read_book.prefs.head_foot import render_head_foot
 from read_book.prefs.scrolling import (
     MIN_SCROLL_SPEED_AUTO as SCROLL_SPEED_STEP, change_scroll_speed
@@ -570,8 +573,11 @@ class View:
         )
 
     def bump_font_size(self, data):
-        delta = 2 if data.increase else -2
-        change_font_size_by(delta)
+        mult = 1 if data.increase else -1
+        frac = 0.2
+        if runtime.is_standalone_viewer:
+            frac = current_zoom_step_size() / 100
+        change_font_size_by(mult * frac)
 
     def on_show_footnote(self, data):
         self.show_content_popup()