Bug 1783183 - Make overlap between scrolled pages configurable r=dholbert

Differential Revision: https://phabricator.services.mozilla.com/D208733

layout/generic/nsGfxScrollFrame.cpp

@@ -5211,15 +5211,19 @@ nsSize nsHTMLScrollFrame::GetPageScrollAmount() const {
   }
 
   nsSize lineScrollAmount = GetLineScrollAmount();
+  const int32_t maxOverlapPercent = std::clamp(
+      StaticPrefs::toolkit_scrollbox_pagescroll_maxOverlapPercent(), 0, 80);
+  const int32_t maxOverlapLines =
+      std::max(StaticPrefs::toolkit_scrollbox_pagescroll_maxOverlapLines(), 0);
 
-  // The page increment is the size of the page, minus the smaller of
-  // 10% of the size or 2 lines.
-  return nsSize(effectiveScrollPortSize.width -
-                    std::min(effectiveScrollPortSize.width / 10,
-                             2 * lineScrollAmount.width),
-                effectiveScrollPortSize.height -
-                    std::min(effectiveScrollPortSize.height / 10,
-                             2 * lineScrollAmount.height));
+  // The page increment is the size of the page, minus some overlap.
+  return nsSize(
+      effectiveScrollPortSize.width -
+          std::min(effectiveScrollPortSize.width * maxOverlapPercent / 100,
+                   maxOverlapLines * lineScrollAmount.width),
+      effectiveScrollPortSize.height -
+          std::min(effectiveScrollPortSize.height * maxOverlapPercent / 100,
+                   maxOverlapLines * lineScrollAmount.height));
 }
 
 /**

modules/libpref/init/StaticPrefList.yaml

@@ -15382,6 +15382,20 @@
   value: 3
   mirror: always
 
+# The maximum overlap between pages when scrolling with
+# page-up/page-down/spacebar, as a percentage of scrollport size.
+- name: toolkit.scrollbox.pagescroll.maxOverlapPercent
+  type: RelaxedAtomicInt32
+  value: 10
+  mirror: always
+
+# The maximum overlap between pages when scrolling with
+# page-up/page-down/spacebar, in lines.
+- name: toolkit.scrollbox.pagescroll.maxOverlapLines
+  type: RelaxedAtomicInt32
+  value: 2
+  mirror: always
+
 # The lateWriteChecksStage and fastShutdownStage below represent the stage
 # of shutdown after which we (for lateWriteChecksStage) crash / gather
 # telemetry data on file writes, or (for fastShutdownStage) we call _exit(0).