From 37472c71ec978325739f9eeb3218d13e620a967e Mon Sep 17 00:00:00 2001
From: Kyle Isom <kyle@imap.cc>
Date: Thu, 4 Dec 2025 15:18:02 -0800
Subject: [PATCH] Fix UI cursor positioning issues.

Accurately recompute cursor position to prevent drift in terminal and GUI renderers.
---
 GUIRenderer.cc      | 14 +++++++++++++-
 TerminalRenderer.cc | 28 ++++++++++++++++++++++++----
 2 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/GUIRenderer.cc b/GUIRenderer.cc
index fd54814..2d8fa91 100644
--- a/GUIRenderer.cc
+++ b/GUIRenderer.cc
@@ -455,7 +455,19 @@ GUIRenderer::Draw(Editor &ed)
 				}
 				// Convert to viewport x by subtracting horizontal col offset
 				std::size_t rx_viewport = (rx_abs > coloffs_now) ? (rx_abs - coloffs_now) : 0;
-				ImVec2 p0 = ImVec2(line_pos.x + static_cast<float>(rx_viewport) * space_w, line_pos.y);
+				// For proportional fonts (Linux GUI), avoid accumulating drift by computing
+				// the exact pixel width of the expanded substring up to the cursor.
+				// expanded contains the line with tabs expanded to spaces and is what we draw.
+				float cursor_px = 0.0f;
+				if (rx_viewport > 0 && coloffs_now < expanded.size()) {
+					std::size_t start = coloffs_now;
+					std::size_t end = std::min(expanded.size(), start + rx_viewport);
+					// Measure substring width in pixels
+					ImVec2 sz = ImGui::CalcTextSize(expanded.c_str() + start,
+									expanded.c_str() + end);
+					cursor_px = sz.x;
+				}
+				ImVec2 p0 = ImVec2(line_pos.x + cursor_px, line_pos.y);
 				ImVec2 p1 = ImVec2(p0.x + space_w, p0.y + line_h);
 				ImU32 col = IM_COL32(200, 200, 255, 128); // soft highlight
 				ImGui::GetWindowDrawList()->AddRectFilled(p0, p1, col);
diff --git a/TerminalRenderer.cc b/TerminalRenderer.cc
index 0dee769..52d7c08 100644
--- a/TerminalRenderer.cc
+++ b/TerminalRenderer.cc
@@ -294,11 +294,31 @@ TerminalRenderer::Draw(Editor &ed)
 			clrtoeol();
 		}
 
-		// Place terminal cursor at logical position accounting for tabs and coloffs
+		// Place terminal cursor at logical position accounting for tabs and coloffs.
+		// Recompute the rendered X using the same logic as the drawing loop to avoid
+		// any drift between the command-layer computation and the terminal renderer.
 		std::size_t cy = buf->Cury();
-		std::size_t rx = buf->Rx(); // render x computed by command layer
-		int cur_y      = static_cast<int>(cy) - static_cast<int>(buf->Rowoffs());
-		int cur_x      = static_cast<int>(rx) - static_cast<int>(buf->Coloffs());
+		std::size_t cx = buf->Curx();
+		int cur_y = static_cast<int>(cy) - static_cast<int>(buf->Rowoffs());
+		std::size_t rx_recomputed = 0;
+		if (cy < lines.size()) {
+			const std::string line_for_cursor = static_cast<std::string>(lines[cy]);
+			std::size_t src_i_cur = 0;
+			std::size_t render_col_cur = 0;
+			while (src_i_cur < line_for_cursor.size() && src_i_cur < cx) {
+				unsigned char ccur = static_cast<unsigned char>(line_for_cursor[src_i_cur]);
+				if (ccur == '\t') {
+					std::size_t next_tab = tabw - (render_col_cur % tabw);
+					render_col_cur += next_tab;
+					++src_i_cur;
+				} else {
+					++render_col_cur;
+					++src_i_cur;
+				}
+			}
+			rx_recomputed = render_col_cur;
+		}
+		int cur_x = static_cast<int>(rx_recomputed) - static_cast<int>(buf->Coloffs());
 		if (cur_y >= 0 && cur_y < content_rows && cur_x >= 0 && cur_x < cols) {
 			// remember where to leave the terminal cursor after status is drawn
 			saved_cur_y = cur_y;