fix: Disable hl-line-mode in chat buffer to prevent scroll oscillation#100
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fix: Disable hl-line-mode in chat buffer to prevent scroll oscillation#100
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The combination of hl-line-mode's post-command-hook overlay mutation, invisible text (markdown-hide-markup), and variable line heights (heading faces) triggers a redisplay feedback loop in GUI Emacs — the window oscillates ±25 lines when scrolling past headings. This is a known class of bugs in hl-line.el going back to 2004. Emacs 31 fixes it for global-hl-line-mode by moving to pre-redisplay-functions (commit 40a22ced147), but local hl-line-mode still uses post-command-hook on all versions. Disable both local and global hl-line-mode in chat-mode using the documented opt-out mechanism (hl-line.el line 50). Add regression test following the existing chat-mode property test pattern.
dnouri
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#100) The combination of hl-line-mode's post-command-hook overlay mutation, invisible text (markdown-hide-markup), and variable line heights (heading faces) triggers a redisplay feedback loop in GUI Emacs — the window oscillates ±25 lines when scrolling past headings. This is a known class of bugs in hl-line.el going back to 2004. Emacs 31 fixes it for global-hl-line-mode by moving to pre-redisplay-functions (commit 40a22ced147), but local hl-line-mode still uses post-command-hook on all versions. Disable both local and global hl-line-mode in chat-mode using the documented opt-out mechanism (hl-line.el line 50). Add regression test following the existing chat-mode property test pattern.
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Problem
In GUI Emacs, scrolling through the chat buffer with
global-hl-line-modeactive causes the window to jump ~27 lines and oscillate between two positions. Three factors must combine to trigger it:hl-line-mode(overlay mutation onpost-command-hook)markdown-hide-markup)Background
hl-line-modecallsmove-overlayinpost-command-hook, which setsprevent_redisplay_optimizations_pin the C display engine. This forces a fullwindow-startrecalculation viatry_to_scroll— but with invisible text and variable pixel heights, the engine picks a different position each frame, creating an oscillation loop. Terminal Emacs is unaffected because all lines have identical pixel height.This is a known bug class going back to 2004. Emacs 31 fixes it for
global-hl-line-modeby moving topre-redisplay-functions(commit40a22ced147), but localhl-line-modestill uses the buggy path on all versions.Fix
Disable both local and global
hl-line-modeinpi-coding-agent-chat-modeusing the documented opt-out. Users lose line highlighting in the chat buffer — acceptable for a buffer that was unusable with it enabled.Changes
pi-coding-agent.el: Disable hl-line in chat-mode bodytest/pi-coding-agent-test.el: Regression test following existing chat-mode property test pattern424/424 tests pass.