feat: add pixel-precision scrolling for terminal output by forketyfork · Pull Request #271 · forketyfork/architect

forketyfork · 2026-03-16T12:13:00Z

Replace line-by-line scrolling with sub-pixel smooth scrolling.
Instead of jumping whole lines on each scroll event, pixel offsets
accumulate and flush to ghostty-vt only when a full cell height is
reached. The renderer applies the fractional pixel offset with SDL
clip rects and renders an extra row to fill gaps.

Key changes:

Add scroll_pixel_offset field to SessionViewState
scrollSession and updateScrollInertia work in pixel space
Renderer shifts rows by pixel offset with content clipping
Scrollbar metrics include fractional offset for smooth thumb
Inertia snap-to-line when velocity decays below threshold

https://claude.ai/code/session_01LdvTgG5JiFGXQcJkpa3eN6

Replace line-by-line scrolling with sub-pixel smooth scrolling. Instead of jumping whole lines on each scroll event, pixel offsets accumulate and flush to ghostty-vt only when a full cell height is reached. The renderer applies the fractional pixel offset with SDL clip rects and renders an extra row to fill gaps. Key changes: - Add scroll_pixel_offset field to SessionViewState - scrollSession and updateScrollInertia work in pixel space - Renderer shifts rows by pixel offset with content clipping - Scrollbar metrics include fractional offset for smooth thumb - Inertia snap-to-line when velocity decays below threshold https://claude.ai/code/session_01LdvTgG5JiFGXQcJkpa3eN6

Copilot

Pull request overview

Implements pixel-precision (sub-line) scrolling for terminal scrollback by accumulating fractional offsets and only committing whole-row scrolls to ghostty-vt, while rendering content with a per-frame pixel shift and updating scrollbar metrics to reflect fractional positions.

Changes:

Add scroll_pixel_offset to SessionViewState and reset it with other scroll state.
Convert wheel + inertia scrolling to accumulate pixel offsets and flush whole lines to ghostty-vt, including an inertia snap-to-line behavior.
Update renderer to shift terminal rows by the pixel offset (with clipping + extra row) and include fractional offset in scrollbar thumb metrics.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/ui/session_view_state.zig	Adds/clears new scroll pixel offset state.
src/ui/components/session_interaction.zig	Routes scroll + inertia through pixel-offset accumulation and snapping.
src/render/renderer.zig	Applies pixel-offset rendering shift, clipping, extra-row fill, and fractional scrollbar thumb.

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src/ui/components/session_interaction.zig

+fn scrollSession(session: *SessionState, view: *SessionViewState, delta: isize, cell_height: c_int, now: i64) void {
    if (!session.spawned) return;
+    if (cell_height <= 0) return;



src/ui/components/session_interaction.zig

+fn scrollSession(session: *SessionState, view: *SessionViewState, delta: isize, cell_height: c_int, now: i64) void {
    if (!session.spawned) return;
+    if (cell_height <= 0) return;

    view.last_scroll_time = now;
    view.scroll_remainder = 0.0;
    view.scroll_inertia_allowed = true;

+    const pixel_delta = @as(f32, @floatFromInt(delta)) * @as(f32, @floatFromInt(cell_height));
+    applyPixelScroll(session, view, pixel_delta, cell_height, now);
+
+    const sensitivity: f32 = 0.08;
+    view.scroll_velocity += @as(f32, @floatFromInt(delta)) * sensitivity;
+    view.scroll_velocity = std.math.clamp(view.scroll_velocity, -max_scroll_velocity, max_scroll_velocity);


src/ui/components/session_interaction.zig

+        // Snap to nearest line boundary when inertia stops
+        if (view.scroll_pixel_offset != 0.0) {
+            const cell_h_f: f32 = @floatFromInt(cell_height);
+            if (session.terminal) |*terminal| {
+                var pages = &terminal.screens.active.pages;
+                if (view.scroll_pixel_offset >= cell_h_f * 0.5) {
+                    // Closer to next line up: commit one more scroll line
+                    pages.scroll(.{ .delta_row = -1 });
+                }
+                view.is_viewing_scrollback = (pages.viewport != .active);
+            }
+            view.scroll_pixel_offset = 0.0;
+            if (!view.is_viewing_scrollback) {
+                view.scroll_pixel_offset = 0.0;
+            }
+            session.markDirty();
+        }


src/ui/components/session_interaction.zig

+    // velocity is in lines/unit; convert to pixel delta
+    const scroll_lines = view.scroll_velocity * delta_time_s * reference_fps + view.scroll_remainder;
+    const pixel_delta = scroll_lines * cell_h_f;

-        if (scroll_lines != 0) {
-            var pages = &terminal.screens.active.pages;
-            pages.scroll(.{ .delta_row = scroll_lines });
-            view.is_viewing_scrollback = (pages.viewport != .active);
-            view.terminal_scrollbar.noteActivity(now_ms);
-            session.markDirty();
-        }
-
-        view.scroll_remainder = scroll_amount - @as(f32, @floatFromInt(scroll_lines));
+    if (pixel_delta != 0.0) {
+        applyPixelScroll(session, view, pixel_delta, cell_height, now_ms);
    }

+    // Track remainder in line-space for next frame
+    view.scroll_remainder = 0.0;
+


src/render/renderer.zig

+    // Pixel-precision scroll: shift content by sub-line pixel offset
+    const pixel_offset: f32 = if (view.is_viewing_scrollback) view.scroll_pixel_offset else 0.0;
+    const pixel_offset_int: c_int = @intFromFloat(pixel_offset);
+    const origin_y: c_int = origin_y_base - pixel_offset_int;

    const max_cols_fit: usize = @intCast(@max(0, @divFloor(drawable_w, cell_width_actual)));
    const max_rows_fit: usize = @intCast(@max(0, @divFloor(drawable_h, cell_height_actual)));
    const visible_cols: usize = @min(@as(usize, term_cols), max_cols_fit);
-    const visible_rows: usize = @min(@as(usize, term_rows), max_rows_fit);
+    // Render one extra row when pixel offset is non-zero to fill the gap at the bottom.
+    // The extra row may not have data (getCell returns null), in which case it's skipped
+    // and the background color (already filled) provides a seamless appearance.
+    const extra_row: usize = if (pixel_offset_int > 0) 1 else 0;
+    const render_rows: usize = @min(@as(usize, term_rows) + extra_row, max_rows_fit + extra_row);
+


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chatgpt-codex-connector · 2026-03-16T12:20:01Z

src/ui/components/session_interaction.zig


                        if (!forwarded) {
-                            scrollSession(session, view, scroll_delta, host.now_ms);
+                            scrollSession(session, view, scroll_delta, self.font.cell_height, host.now_ms);


Pass scaled cell height into smooth scroll physics

Wheel and inertia scrolling now always use self.font.cell_height as the pixel unit, but sessions in Grid/Expanding/Collapsing are rendered with a smaller per-session cell_height_actual (renderer.zig scales rows by scale). This makes scroll_pixel_offset larger than a rendered row in those modes, so content can shift by multiple visual rows while only one extra row is rendered, causing visible jumps/blank bands when scrolling non-fullscreen tiles. Compute scroll pixel deltas with the active session’s rendered cell height (or store offsets in row fractions) instead of the unscaled font height.

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chatgpt-codex-connector · 2026-03-16T12:20:01Z

src/render/renderer.zig

    const metrics = scrollbar.Metrics.init(
        @as(f32, @floatFromInt(bar.total)),
-        @as(f32, @floatFromInt(bar.offset)),
+        @as(f32, @floatFromInt(bar.offset)) + fractional_offset,


Keep scrollbar hit-testing aligned with fractional thumb offset

The renderer now positions the thumb using bar.offset + fractional_offset, but input-side scrollbar layout (terminalScrollbarContext) still uses only integer bar.offset. When scroll_pixel_offset is non-zero during smooth/inertial scrollback, the drawn thumb and the hover/drag geometry are computed from different offsets, so pointer hit-testing can miss or jump relative to what is on screen (most noticeable with short scrollback ranges). Use the same fractional offset term in the interaction metrics.

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forketyfork requested a review from Copilot March 16, 2026 12:13

Copilot started reviewing on behalf of forketyfork March 16, 2026 12:14 View session

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feat: add pixel-precision scrolling for terminal output#271

feat: add pixel-precision scrolling for terminal output#271
forketyfork wants to merge 1 commit intomainfrom
claude/pixel-precision-scroll-qcn81

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