Debug cursor scroll into view behavior

Helge Sverre
Helge Sverre Hessevik Liseth • @helgesverre
 5mo ago
token
main
Smart (Opus 4.6)
Unlisted
investigate how cursor scroll into view is implemented in this project, there is a bug where if i click somewhere below the midpointish in an editor area it automatically scrolls the cursor to the middle, this should only occur if im clicking at the last visible line in the viewport or navigating up/down out of the safe area or my cursor is outside the viewport (eg click to palce cursor, scroll down past it in a long file or up it) and when mouse movement is triggered it should focus back to cursor) there is some kind of bug in this system, review it thuroughly to try to identify where this comes from, consult oracle for guidance and librarian to look at how this is implemented by other editors and compare it to ours

Summary

Root cause: When new editors are created (EditorState::new() via open_file_in_new_tab and split_group), they get viewport.visible_lines = 25 (the hardcoded default). resize() is only called on WindowEvent::Resized — not when new editors are created. So new editors retain visible_lines = 25 until the window is next resized.

Why midpoint: On a typical window showing ~45 lines, the safe zone boundary is at line 23 (= 25 - 2). Clicking on line 24+ triggers scrolling. Line 24 is roughly the visual midpoint of a 45-line viewport.

Fix approach — two options:

  1. Quick fix: Call model.recalculate_viewports() (or model.resize(w, h)) after creating new editors in new_tab_in_focused_group, open_file_in_new_tab, and split_group.

  2. Robust fix: Call editor_area.sync_all_viewports(line_height, char_width) after compute_layout_scaled in the render path, so viewport dimensions always stay in sync with actual group dimensions. This also fixes the secondary issue for split views where groups can be different sizes than what resize() computes.

Bonus consideration: For mouse clicks specifically, you may want to skip ensure_cursor_visible entirely (or use a no-padding mode), since the clicked position is by definition already visible. VS Code and other editors don't apply scroll margins to mouse clicks — only to keyboard navigation.

Want me to implement the fix?

yes, lets do 2, and the bonus consideration as well, lets additionally expand tests to cover this case (tdd out the fix) so we catch regressions of this behaviour

All clean. Here's a summary of the three fixes:

Changes Made

Fix 1: Mouse clicks don't apply scroll padding (src/update/editor.rs, src/model/editor.rs, src/model/editor_area.rs, src/model/mod.rs)

  • Added ensure_cursor_visible_no_padding() — only scrolls if cursor is completely outside the viewport, ignoring scroll_padding
  • SetCursorPosition handler now uses this method instead of ensure_cursor_visible()
  • Clicking on any visible line (including first/last) never scrolls

Fix 2: sync_all_viewports subtracts tab bar height (src/model/editor_area.rs, src/bin/screenshot.rs)

  • Added tab_bar_height parameter to sync_all_viewports()
  • Now correctly passes it to compute_visible_lines() so the text content area height is used, not the full group rect

Fix 3: New editors get correct viewport dimensions (src/update/layout.rs)

  • Added sync_viewports() helper called after NewTab, OpenFileInNewTab, SplitFocused, and SplitGroup
  • New editors now inherit correct visible_lines from their group rect instead of keeping the default 25

Tests added (7 new tests in tests/scrolling.rs)

  • Click on visible line → no scroll
  • Click on last/first visible line → no scroll (even with padding)
  • Click off-screen above/below → scrolls correctly
  • sync_all_viewports correctly subtracts tab bar height
  • New editor tab gets correct viewport size
commit changes
Explored 1 search

There are unrelated changes in the worktree. I'll only stage the files I modified:

$ git add \ src/model/editor.rs \ src/model/editor_area.rs \ src/model/mod.rs \ src/update/editor.rs \ src/update/layout.rs \ src/bin/screenshot.rs \ tests/scrolling.rs
Explored 1 search
$ git commit -m "fix: prevent unwanted viewport scroll on mouse click Mouse clicks on visible lines no longer trigger scroll-into-view with padding. Previously, clicking near viewport edges (or past the midpoint on new editors with default visible_lines=25) would scroll the viewport. Three fixes: - SetCursorPosition uses ensure_cursor_visible_no_padding(), which only scrolls when the cursor is completely outside the viewport bounds - sync_all_viewports now subtracts tab_bar_height from group rect height for correct visible_lines computation - New editors (new tab, open file, split) sync viewport dimensions from their group rect instead of keeping the default 25 lines"

Committed as 9f08daf on main.