fix: overflow when computing scrollbar rect (#6547)

Co-authored-by: Kasia <zneix@zneix.eu>

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Minor: Added link support to plugin message API. (#6386)
 - Minor: Added a description for the logging option under moderation tab. (#6514)
 - Bugfix: Expose the "Extra extension IDs" setting on non-Windows systems too. (#6509)
+- Bugfix: Fixed scrollbar rect cmputation potentially resulting in overflows. (#6547)
 - Dev: Update release documentation. (#6498)
 - Dev: Make code sanitizers opt in with the `CHATTERINO_SANITIZER_SUPPORT` CMake option. After that's enabled, use the `SANITIZE_*` flag to enable individual sanitizers. (#6493)
 - Dev: Remove unused QTextCodec includes. (#6487)

src/widgets/Scrollbar.cpp

@@ -465,16 +465,17 @@ void Scrollbar::updateScroll()
 {
     this->trackHeight_ = this->height() - MIN_THUMB_HEIGHT - 1;
 
-    auto div = std::max<qreal>(0.0000001, this->maximum_ - this->minimum_);
+    auto nMessages = std::max<qreal>(1.0, this->maximum_ - this->minimum_);
 
-    this->thumbRect_ =
-        QRect(0,
-              static_cast<int>((this->getRelativeCurrentValue()) / div *
-                               this->trackHeight_) +
-                  1,
-              this->width(),
-              static_cast<int>(this->pageSize_ / div * this->trackHeight_) +
-                  MIN_THUMB_HEIGHT);
+    this->thumbRect_ = QRect{
+        0,
+        static_cast<int>((this->getRelativeCurrentValue()) / nMessages *
+                         this->trackHeight_) +
+            1,
+        this->width(),
+        static_cast<int>(this->pageSize_ / nMessages * this->trackHeight_) +
+            MIN_THUMB_HEIGHT,
+    };
 
     this->update();
 }

src/widgets/Scrollbar.hpp

@@ -35,18 +35,21 @@ class ChannelView;
 /// index is computed by `scrollbarPos - minimum` - thus a scrollbar position
 /// of a message is at `index + minimum.
 ///
-/// @cond src-only
-///
 /// The following illustrates a scrollbar in a channel view with seven
 /// messages. The scrollbar is at the bottom. No animation is active, thus
 /// `currentValue = desiredValue`.
 ///
+/// ```text
 ///                ┌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┐←╌╌╌ minimum
-///                  Alice: This message is quite           = 0
+///                  Bob: Ugh, I can't see this              = 0
+///                  message, it's outside of my
+///                  ChannelView
+///                ├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤
+///                  Alice: Your message is quite
 ///             ┬  ╭─────────────────────────────────╮←╮
 ///             │  │ long, so it gets wrapped        │ ┆
 ///             │  ├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤ ╰╌╌╌ currentValue
-///             │  │ Bob: are you sure?              │       = 0.5
+///             │  │ Bob: are you sure?              │       = 1.5
 ///             │  ├╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┤       = desiredValue
 /// pageSize ╌╌╌┤  │ Alice: Works for me... try for  │       = maximum
 ///  = 6.5      │  │ yourself                        │         - pageSize
@@ -63,8 +66,8 @@ class ChannelView;
 ///             ┴╭→╰─────────────────────────────────╯
 ///              ┆
 ///           maximum
-///            = 7
-/// @endcond
+///            = 8
+/// ```
 ///
 /// When messages are added at the bottom, both maximum and minimum are offset
 /// by 1 and after a layout, the desired value is updated, causing the content