Forward touch scroll events as terminal input

Dispatches synthetic WheelEvents into hterm's existing onMouse_
pipeline so touch scroll gestures reach programs that listen for
them. hterm handles mouse reporting (VT) and alternate screen
arrow keys (DECSET 1007) automatically.

On the primary screen with mouse reporting disabled, the existing
visual scrollback behavior is preserved unchanged.

Fixes #2375
Fixes #2537

Author: splch

app/TerminalView.m

@@ -31,7 +31,9 @@ - (void)userContentController:(WKUserContentController *)userContentController d
 }
 @end
 
-@interface TerminalView ()
+@interface TerminalView () {
+    CGFloat _prevScrollY;
+}
 
 @property (nonatomic) NSMutableArray<UIKeyCommand *> *keyCommands;
 @property ScrollbarView *scrollbarView;
@@ -104,6 +106,7 @@ - (void)setTerminal:(Terminal *)terminal {
     }
 
     _terminal = terminal;
+    _prevScrollY = 0;
     [_terminal addObserver:self forKeyPath:@"loaded" options:NSKeyValueObservingOptionInitial context:nil];
     if (_terminal.loaded)
         [self installTerminalView];
@@ -268,7 +271,10 @@ - (void)userContentController:(WKUserContentController *)userContentController d
 }
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView {
-    [self.terminal.webView evaluateJavaScript:[NSString stringWithFormat:@"exports.newScrollTop(%f)", scrollView.contentOffset.y] completionHandler:nil];
+    CGFloat newY = scrollView.contentOffset.y;
+    CGFloat delta = newY - _prevScrollY;
+    _prevScrollY = newY;
+    [self.terminal.webView evaluateJavaScript:[NSString stringWithFormat:@"exports.handleScroll(%f, %f)", newY, delta] completionHandler:nil];
 }
 
 - (void)setKeyboardAppearance:(UIKeyboardAppearance)keyboardAppearance {

app/terminal/term.js

@@ -110,12 +110,38 @@ term.scrollPort_.onTouch = (e) => {
 };
 // Scroll to bottom wrapper
 exports.scrollToBottom = () => term.scrollEnd();
-// Set scroll position
-exports.newScrollTop = (y) => {
-    // two lines instead of one because the value you read out of scrollTop can be different from the value you write into it
+// Set scroll position and optionally forward delta as WheelEvents into hterm's
+// onMouse_ pipeline for mouse reporting (VT) and alternate screen arrow keys.
+let scrollDeltaRemainder = 0;
+exports.handleScroll = (y, delta) => {
     term.scrollPort_.screen_.scrollTop = y;
     lastScrollTop = term.scrollPort_.screen_.scrollTop;
+
+    if (delta === 0) return;
+    if (term.vt.mouseReport === term.vt.MOUSE_REPORT_DISABLED && term.isPrimaryScreen())
+        return;
+
+    const charH = term.scrollPort_.characterSize.height;
+    if (!charH) return;
+
+    scrollDeltaRemainder += delta;
+    const lines = Math.trunc(scrollDeltaRemainder / charH);
+    if (lines === 0) return;
+    scrollDeltaRemainder -= lines * charH;
+
+    const abs = Math.abs(lines);
+    const down = lines > 0;
+    for (let i = 0; i < abs; i++) {
+        term.scrollPort_.screen_.dispatchEvent(new WheelEvent('wheel', {
+            deltaY: down ? 1 : -1,
+            deltaMode: WheelEvent.DOM_DELTA_LINE,
+            bubbles: true,
+            cancelable: true,
+        }));
+    }
 };
+// Keep old name for compatibility
+exports.newScrollTop = (y) => exports.handleScroll(y, 0);
 
 // Send scroll height and position to native code
 let lastScrollHeight, lastScrollTop;
@@ -154,6 +180,7 @@ exports.getCharacterSize = () => {
 };
 
 exports.clearScrollback = () => term.clearScrollback();
+
 exports.setUserGesture = () => term.accessibilityReader_.hasUserGesture = true;
 
 hterm.openUrl = (url) => native.openLink(url);