Fix slow scrolling in dock panels

Commit f688bcd1f1 fixed slow scrolling with xf86-input-libinput on DolphinView.

However the commit also exposed a bug in the Dolphin scrolling
algorithm, which was previously hidden. This resulted in slow
scrolling in dock panels (Places and Folders), with both
xf86-input-evdev and xf86-input-libinput drivers, as well as libinput on
Wayland.

KItemListContainer::updateScrollOffsetScrollBar() relied on the view's
itemSize() method to compute the scrollbar's singleStep, but this QSize
was invalid for the dock panels' views.

We use a new itemSizeHint() method instead, which is always valid and
also adapts to the current icon size set in the view.

BUG: 365968
FIXED-IN: 16.12.0
REVIEW: 129409

1 files changed, 13 insertions, 0 deletions

diff --git a/src/kitemviews/private/kitemlistsizehintresolver.cpp b/src/kitemviews/private/kitemlistsizehintresolver.cpp
index 1d8067026..02f1865b3 100644
--- a/src/kitemviews/private/kitemlistsizehintresolver.cpp
+++ b/src/kitemviews/private/kitemlistsizehintresolver.cpp
@@ -25,6 +25,7 @@ KItemListSizeHintResolver::KItemListSizeHintResolver(const KItemListView* itemLi
     m_itemListView(itemListView),
     m_logicalHeightHintCache(),
     m_logicalWidthHint(0.0),
+    m_logicalHeightHint(0.0),
     m_needsResolving(false)
 {
 }
@@ -33,6 +34,12 @@ KItemListSizeHintResolver::~KItemListSizeHintResolver()
 {
 }
 
+QSizeF KItemListSizeHintResolver::maxSizeHint()
+{
+    updateCache();
+    return QSizeF(m_logicalWidthHint, m_logicalHeightHint);
+}
+
 QSizeF KItemListSizeHintResolver::sizeHint(int index)
 {
     updateCache();
@@ -149,6 +156,12 @@ void KItemListSizeHintResolver::updateCache()
 {
     if (m_needsResolving) {
         m_itemListView->calculateItemSizeHints(m_logicalHeightHintCache, m_logicalWidthHint);
+        // Set logical height as the max cached height (if the cache is not empty).
+        if (m_logicalHeightHintCache.isEmpty()) {
+            m_logicalHeightHint = 0.0;
+        } else {
+            m_logicalHeightHint = *std::max_element(m_logicalHeightHintCache.begin(), m_logicalHeightHintCache.end());
+        }
         m_needsResolving = false;
     }
 }