Make sure scrollbars have the right size initially

We refresh the view after reading just enough to fill it, so that we see the
initial content as quickly as possible, but then we continue reading enough
lines so that we can tell how long the scrollbar needs to be, and then we
refresh again. This can result in slight flicker of the scrollbar when it is
first drawn with a bigger size and then jumps to a smaller size; however, that's
a good tradeoff for a solution that provides both good speed and accuracy.

3 files changed, 32 insertions, 8 deletions

diff --git a/pkg/gui/pty.go b/pkg/gui/pty.go
index 4a87c98da..fe53697c8 100644
--- a/pkg/gui/pty.go
+++ b/pkg/gui/pty.go
@@ -55,9 +55,6 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 
 	cmd.Env = append(cmd.Env, "GIT_PAGER="+pager)
 
-	_, height := view.Size()
-	_, oy := view.Origin()
-
 	manager := gui.getManager(view)
 
 	var ptmx *os.File
@@ -82,7 +79,8 @@ func (gui *Gui) newPtyTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 		gui.Mutexes.PtyMutex.Unlock()
 	}
 
-	if err := manager.NewTask(manager.NewCmdTask(start, prefix, height+oy+10, onClose), cmdStr); err != nil {
+	linesToRead := gui.linesToReadFromCmdTask(view)
+	if err := manager.NewTask(manager.NewCmdTask(start, prefix, linesToRead, onClose), cmdStr); err != nil {
 		return err
 	}
 
diff --git a/pkg/gui/tasks_adapter.go b/pkg/gui/tasks_adapter.go
index 33aa09eb9..69b64d7b1 100644
--- a/pkg/gui/tasks_adapter.go
+++ b/pkg/gui/tasks_adapter.go
@@ -16,9 +16,6 @@ func (gui *Gui) newCmdTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 		cmdStr,
 	).Debug("RunCommand")
 
-	_, height := view.Size()
-	_, oy := view.Origin()
-
 	manager := gui.getManager(view)
 
 	start := func() (*exec.Cmd, io.Reader) {
@@ -35,7 +32,8 @@ func (gui *Gui) newCmdTask(view *gocui.View, cmd *exec.Cmd, prefix string) error
 		return cmd, r
 	}
 
-	if err := manager.NewTask(manager.NewCmdTask(start, prefix, height+oy+10, nil), cmdStr); err != nil {
+	linesToRead := gui.linesToReadFromCmdTask(view)
+	if err := manager.NewTask(manager.NewCmdTask(start, prefix, linesToRead, nil), cmdStr); err != nil {
 		gui.c.Log.Error(err)
 	}
 
diff --git a/pkg/gui/view_helpers.go b/pkg/gui/view_helpers.go
index c51942b03..8f2055245 100644
--- a/pkg/gui/view_helpers.go
+++ b/pkg/gui/view_helpers.go
@@ -6,6 +6,7 @@ import (
 	"github.com/jesseduffield/gocui"
 	"github.com/jesseduffield/lazygit/pkg/gui/keybindings"
 	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/tasks"
 	"github.com/jesseduffield/lazygit/pkg/utils"
 	"github.com/spkg/bom"
 )
@@ -15,6 +16,33 @@ func (gui *Gui) resetOrigin(v *gocui.View) error {
 	return v.SetOrigin(0, 0)
 }
 
+// Returns the number of lines that we should read initially from a cmd task so
+// that the scrollbar has the correct size, along with the number of lines after
+// which the view is filled and we can do a first refresh.
+func (gui *Gui) linesToReadFromCmdTask(v *gocui.View) tasks.LinesToRead {
+	_, height := v.Size()
+	_, oy := v.Origin()
+
+	linesForFirstRefresh := height + oy + 10
+
+	// We want to read as many lines initially as necessary to let the
+	// scrollbar go to its minimum height, so that the scrollbar thumb doesn't
+	// change size as you scroll down.
+	minScrollbarHeight := 2
+	linesToReadForAccurateScrollbar := height*(height-1)/minScrollbarHeight + oy
+
+	// However, cap it at some arbitrary max limit, so that we don't get
+	// performance problems for huge monitors or tiny font sizes
+	if linesToReadForAccurateScrollbar > 5000 {
+		linesToReadForAccurateScrollbar = 5000
+	}
+
+	return tasks.LinesToRead{
+		Total:               linesToReadForAccurateScrollbar,
+		InitialRefreshAfter: linesForFirstRefresh,
+	}
+}
+
 func (gui *Gui) cleanString(s string) string {
 	output := string(bom.Clean([]byte(s)))
 	return utils.NormalizeLinefeeds(output)