Add ScrollOffMargin user config

When set to a non-zero value, views will scroll when the selection gets this
close to the top or bottom of the view.

5 files changed, 261 insertions, 0 deletions

diff --git a/pkg/config/user_config.go b/pkg/config/user_config.go
index 9eb8e028a..a217d8099 100644
--- a/pkg/config/user_config.go
+++ b/pkg/config/user_config.go
@@ -31,6 +31,7 @@ type GuiConfig struct {
 	BranchColors              map[string]string  `yaml:"branchColors"`
 	ScrollHeight              int                `yaml:"scrollHeight"`
 	ScrollPastBottom          bool               `yaml:"scrollPastBottom"`
+	ScrollOffMargin           int                `yaml:"scrollOffMargin"`
 	MouseEvents               bool               `yaml:"mouseEvents"`
 	SkipDiscardChangeWarning  bool               `yaml:"skipDiscardChangeWarning"`
 	SkipStashWarning          bool               `yaml:"skipStashWarning"`
@@ -418,6 +419,7 @@ func GetDefaultConfig() *UserConfig {
 		Gui: GuiConfig{
 			ScrollHeight:             2,
 			ScrollPastBottom:         true,
+			ScrollOffMargin:          2,
 			MouseEvents:              true,
 			SkipDiscardChangeWarning: false,
 			SkipStashWarning:         false,
diff --git a/pkg/gui/controllers/list_controller.go b/pkg/gui/controllers/list_controller.go
index fb6d8736a..cdaea413a 100644
--- a/pkg/gui/controllers/list_controller.go
+++ b/pkg/gui/controllers/list_controller.go
@@ -82,6 +82,12 @@ func (self *ListController) handleLineChange(change int) error {
 	// doing this check so that if we're holding the up key at the start of the list
 	// we're not constantly re-rendering the main view.
 	if before != after {
+		if change == -1 {
+			checkScrollUp(self.context.GetViewTrait(), self.c.UserConfig.Gui.ScrollOffMargin, before, after)
+		} else if change == 1 {
+			checkScrollDown(self.context.GetViewTrait(), self.c.UserConfig.Gui.ScrollOffMargin, before, after)
+		}
+
 		return self.context.HandleFocus(types.OnFocusOpts{})
 	}
 
diff --git a/pkg/gui/controllers/patch_explorer_controller.go b/pkg/gui/controllers/patch_explorer_controller.go
index dd19d08db..83c8633a7 100644
--- a/pkg/gui/controllers/patch_explorer_controller.go
+++ b/pkg/gui/controllers/patch_explorer_controller.go
@@ -159,13 +159,25 @@ func (self *PatchExplorerController) GetMouseKeybindings(opts types.KeybindingsO
 }
 
 func (self *PatchExplorerController) HandlePrevLine() error {
+	before := self.context.GetState().GetSelectedLineIdx()
 	self.context.GetState().CycleSelection(false)
+	after := self.context.GetState().GetSelectedLineIdx()
+
+	if self.context.GetState().SelectingLine() {
+		checkScrollUp(self.context.GetViewTrait(), self.c.UserConfig.Gui.ScrollOffMargin, before, after)
+	}
 
 	return nil
 }
 
 func (self *PatchExplorerController) HandleNextLine() error {
+	before := self.context.GetState().GetSelectedLineIdx()
 	self.context.GetState().CycleSelection(true)
+	after := self.context.GetState().GetSelectedLineIdx()
+
+	if self.context.GetState().SelectingLine() {
+		checkScrollDown(self.context.GetViewTrait(), self.c.UserConfig.Gui.ScrollOffMargin, before, after)
+	}
 
 	return nil
 }
diff --git a/pkg/gui/controllers/scroll_off_margin.go b/pkg/gui/controllers/scroll_off_margin.go
new file mode 100644
index 000000000..119f30090
--- /dev/null
+++ b/pkg/gui/controllers/scroll_off_margin.go
@@ -0,0 +1,70 @@
+package controllers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+)
+
+// To be called after pressing up-arrow; checks whether the cursor entered the
+// top scroll-off margin, and so the view needs to be scrolled up one line
+func checkScrollUp(view types.IViewTrait, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) {
+	viewPortStart, viewPortHeight := view.ViewPortYBounds()
+
+	linesToScroll := calculateLinesToScrollUp(
+		viewPortStart, viewPortHeight, scrollOffMargin, lineIdxBefore, lineIdxAfter)
+	if linesToScroll != 0 {
+		view.ScrollUp(linesToScroll)
+	}
+}
+
+// To be called after pressing down-arrow; checks whether the cursor entered the
+// bottom scroll-off margin, and so the view needs to be scrolled down one line
+func checkScrollDown(view types.IViewTrait, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) {
+	viewPortStart, viewPortHeight := view.ViewPortYBounds()
+
+	linesToScroll := calculateLinesToScrollDown(
+		viewPortStart, viewPortHeight, scrollOffMargin, lineIdxBefore, lineIdxAfter)
+	if linesToScroll != 0 {
+		view.ScrollDown(linesToScroll)
+	}
+}
+
+func calculateLinesToScrollUp(viewPortStart int, viewPortHeight int, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) int {
+	// Cap the margin to half the view height. This allows setting the config to
+	// a very large value to keep the cursor always in the middle of the screen.
+	// Use +.5 so that if the height is even, the top margin is one line higher
+	// than the bottom margin.
+	scrollOffMargin = utils.Min(scrollOffMargin, int((float64(viewPortHeight)+.5)/2))
+
+	// Scroll only if the "before" position was visible (this could be false if
+	// the scroll wheel was used to scroll the selected line out of view) ...
+	if lineIdxBefore >= viewPortStart && lineIdxBefore < viewPortStart+viewPortHeight {
+		marginEnd := viewPortStart + scrollOffMargin
+		// ... and the "after" position is within the top margin (or before it)
+		if lineIdxAfter < marginEnd {
+			return marginEnd - lineIdxAfter
+		}
+	}
+
+	return 0
+}
+
+func calculateLinesToScrollDown(viewPortStart int, viewPortHeight int, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) int {
+	// Cap the margin to half the view height. This allows setting the config to
+	// a very large value to keep the cursor always in the middle of the screen.
+	// Use -.5 so that if the height is even, the bottom margin is one line lower
+	// than the top margin.
+	scrollOffMargin = utils.Min(scrollOffMargin, int((float64(viewPortHeight)-.5)/2))
+
+	// Scroll only if the "before" position was visible (this could be false if
+	// the scroll wheel was used to scroll the selected line out of view) ...
+	if lineIdxBefore >= viewPortStart && lineIdxBefore < viewPortStart+viewPortHeight {
+		marginStart := viewPortStart + viewPortHeight - scrollOffMargin - 1
+		// ... and the "after" position is within the bottom margin (or after it)
+		if lineIdxAfter > marginStart {
+			return lineIdxAfter - marginStart
+		}
+	}
+
+	return 0
+}
diff --git a/pkg/gui/controllers/scroll_off_margin_test.go b/pkg/gui/controllers/scroll_off_margin_test.go
new file mode 100644
index 000000000..099da299f
--- /dev/null
+++ b/pkg/gui/controllers/scroll_off_margin_test.go
@@ -0,0 +1,171 @@
+package controllers
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_calculateLinesToScrollUp(t *testing.T) {
+	scenarios := []struct {
+		name                  string
+		viewPortStart         int
+		viewPortHeight        int
+		scrollOffMargin       int
+		lineIdxBefore         int
+		lineIdxAfter          int
+		expectedLinesToScroll int
+	}{
+		{
+			name:                  "before position is above viewport - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         9,
+			lineIdxAfter:          8,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before position is below viewport - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         20,
+			lineIdxAfter:          19,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before and after positions are outside scroll-off margin - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         14,
+			lineIdxAfter:          13,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before outside, after inside scroll-off margin - scroll by 1",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         13,
+			lineIdxAfter:          12,
+			expectedLinesToScroll: 1,
+		},
+		{
+			name:                  "before inside scroll-off margin - scroll by more than 1",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         11,
+			lineIdxAfter:          10,
+			expectedLinesToScroll: 3,
+		},
+		{
+			name:                  "very large scroll-off margin - keep view centered (even viewport height)",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       999,
+			lineIdxBefore:         15,
+			lineIdxAfter:          14,
+			expectedLinesToScroll: 1,
+		},
+		{
+			name:                  "very large scroll-off margin - keep view centered (odd viewport height)",
+			viewPortStart:         10,
+			viewPortHeight:        9,
+			scrollOffMargin:       999,
+			lineIdxBefore:         14,
+			lineIdxAfter:          13,
+			expectedLinesToScroll: 1,
+		},
+	}
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			linesToScroll := calculateLinesToScrollUp(scenario.viewPortStart, scenario.viewPortHeight, scenario.scrollOffMargin, scenario.lineIdxBefore, scenario.lineIdxAfter)
+			assert.Equal(t, scenario.expectedLinesToScroll, linesToScroll)
+		})
+	}
+}
+
+func Test_calculateLinesToScrollDown(t *testing.T) {
+	scenarios := []struct {
+		name                  string
+		viewPortStart         int
+		viewPortHeight        int
+		scrollOffMargin       int
+		lineIdxBefore         int
+		lineIdxAfter          int
+		expectedLinesToScroll int
+	}{
+		{
+			name:                  "before position is above viewport - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         9,
+			lineIdxAfter:          10,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before position is below viewport - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         20,
+			lineIdxAfter:          21,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before and after positions are outside scroll-off margin - don't scroll",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         15,
+			lineIdxAfter:          16,
+			expectedLinesToScroll: 0,
+		},
+		{
+			name:                  "before outside, after inside scroll-off margin - scroll by 1",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         16,
+			lineIdxAfter:          17,
+			expectedLinesToScroll: 1,
+		},
+		{
+			name:                  "before inside scroll-off margin - scroll by more than 1",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       3,
+			lineIdxBefore:         18,
+			lineIdxAfter:          19,
+			expectedLinesToScroll: 3,
+		},
+		{
+			name:                  "very large scroll-off margin - keep view centered (even viewport height)",
+			viewPortStart:         10,
+			viewPortHeight:        10,
+			scrollOffMargin:       999,
+			lineIdxBefore:         15,
+			lineIdxAfter:          16,
+			expectedLinesToScroll: 1,
+		},
+		{
+			name:                  "very large scroll-off margin - keep view centered (odd viewport height)",
+			viewPortStart:         10,
+			viewPortHeight:        9,
+			scrollOffMargin:       999,
+			lineIdxBefore:         14,
+			lineIdxAfter:          15,
+			expectedLinesToScroll: 1,
+		},
+	}
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			linesToScroll := calculateLinesToScrollDown(scenario.viewPortStart, scenario.viewPortHeight, scenario.scrollOffMargin, scenario.lineIdxBefore, scenario.lineIdxAfter)
+			assert.Equal(t, scenario.expectedLinesToScroll, linesToScroll)
+		})
+	}
+}